The experimental production of cyclopia in the fish embryo (Fundulus heteroclitus).код для вставкиСкачать
THE EXPERIMENTAL PIlODUCTION O F CYCLOPIA I N THE FISH EMBRYO (FUNDULUS HETEROCLITUS). BY In a series of experiments on localizatioii and regeneration in the fish embryo it was noted that defects, made in the anterior end of the embryonic shield, often gave rise to cyclopean forms. With the renewed interest in the subject since the publication of the papers by Spemann and Stockard, it seems desirable to indicate this mechanical method by which these forms were produced and to enter somewhat upon the bearing they may have upon the theories of the origin of Cyclopia. Physicians have been interested in Cyclopean monsters for centuries and many theories concerning thcir origin have been advanced. Two groups of theories are to bc recognized, the germinal and the enriromental. Concerning the germinal origin there is no direct or indirect proof such as might be obtained were cyclopean monsters viable and capable of sexual maturity and subsequent re production with the possibility of transmitting to their offspring the same peculiarity, through peculiarities in the germplasm. The very fact that this chance of transmission is eliminated would speak against the germinal origin. There is considerable evidence in favor of the second view, that Cyclopia are due to some modification of the embryo during the early stages of development. The experiments of Spemann and Stockard and likewise of the ones recorded below seem to support this view. Spemann's' experiments consisted in constricting the eggs of triton during the two cell or later stages by a fine thread about the circumference of the first cleavage plane. Double headed monsters, exhibiting rarying degrees of fusion of Weber experimentell erzeugte Doppelbildungen niit cTclopischem Defekt. 2001. Jahrbuch., Supp. VII, 1904. (175) 176 Warren H. Lewis. n FIGS.1, 2.-Showing embryonic shield at operation stnges. The black area shows usual amount of tissue lost by the operation. FIGS.3, 4, 5.-Dorsal, lateral and ventral views of normal head shortly after hatching. FIQ.6.-Experiment ha,. Operation on stage 2 (see Fig. 2). FIGS. 7,8, 9.-Experiment ha,,. Operation on stage 2. Right eye completely absent. FIGS.10, 11, 12.-Experiment ha,. Operation on stage 2. Left eye entirely absent. The Experimental Production of Cplopia. 1;: two or more eyes were often produced. This fusion of the eyes was probably primary before the eye rudiments were recognizable. In StoCkard’s* experiments on fish embryos (Fundulus heteroclitus) , the eggs shortly after fertilization were placed in sea water solution of magnesium chloride with rcsulting production of the cyclopean condition in a large percentage of the eggs. The cyclopean condition i? also primary in these experiments, “the earliest indication of an eye is just as truly cyclopean as it will be later,” S t o ~ k a r d . ~ Neither Spemann’s nor Stockard’s cyclopean monsters can be looked upon as germinal in origin but are truly due to environmental conditions. The additional data from the following experiments extends the possibility of cyclopean monsters depending upon a b normal influences exerted during early embryonic development. These experiments were done at the Marine Biological Laboratory, \Yoodu Hole, Mass., and a few of the typical ones are given below. Such forms were easily reproduced during two succeeding seasons. The experiments were made on the eggs of Fundulus heteroclitns clnriiig the embryonic shield stage. The egg was held with a small pair of forceps and a very fine needle was thrust through the cpg niembrane into the anterior end of the shield; as the needle was withdrawn slight pressure on the forceps caused some of the material of the embryonic shield in the region of the needle prick to be estruded. As the experiments were done under the binocular microscope, it was possible to determine with some degree of accuracy about how much material had escaped. This is indicated in Figs. 1 and 2 by the solid black patch at the anterior end of the ernbryonic shield. The amount of material extruded yaried somewhat in each experiment, the variations becoming more eridcnt during the later stages of development. After the embryonic shield begins to appear, there is w r y little or no regeneration of the central nervous system and defects caused at this time consequently become more and more apparent as development proceeds. *The artificial prodiiction of R single niediau cyclopean eye in the fish enibryo by x~leansof Sea Water Solutions of Magnesinin Chloride. Arch. f. Ent~~iclrlungsxnechanfkder Organisnieii, Bd. 23, 1907. ”Science, 1’01. XSVIII, p. 455. 178 Warren H. Lewis. 16 21 FIGS.13, 14, 15.-Experiment hm,. Operation on stage 1 (see Fig. 1). E p m i n contact i n median plane. FIQS.16, 17, 18.-Experiment h,. Operation on stage 1. Eyes fused, with two lenses and two cup cavities. FIQS.19, 20, 21.-Experiment ha,,. Operation on stage 2. Eyes fused with two lenses and one large cup cavity. YIOS. 22, 23, =.-Experiment hall. Operation on stage 1. Eyes completely fused with one lens and one cup cavity. The Experimental Production of Cyclopia. 179 In a number of instances the material taken out with the needle point was from one side of the anterior end of the embryonic shield with the resulting absence at the time of hatching of the eye on that side. Fig. 6 shows the head of such an embryo, which was operated on at the stage shown in Fig. 2 and killed a few days, after hatching, 15 days after the operation aiid I f days after fertilization. The right eye consists merely of a small bit of retina remaining in the otherwise almost normal brain mall. The left eye is apparently normal as are also the brain and nasal pits. Figs. 7, 8, 0 are from another embryo operated on at the same time and stage as the one shown in Fig. 6, and killed 15 days after the operation. The sections show complete absence of the right eye, but with an otherwise normal brain and head. Figs. 10, 11, and 12 are from another embryo operated on at the same time and stage and killed 15 days later. I n this embryo the left eye is entirely wanting, the forebrain is slightly reduced in size aiid the nasal pits are quite close together. The right eye lies nearer the median plane than normal. See Figs. 3, 4, and 5. Figs. 13, 14 and 15 are from an embryo in which the operation defect was about medial and done at a stage such as seen in Fig. 1. Fifteen days after the operation the embryo was pulled out of the membrane and killed. The two eyes are in contact, but each one is surrounded even at the place of contact with its own pigment layer. Two optic nerves are present and two lenses. The two nasal pits are in contact, but the brain is apparently about normal in size. Figs. 16, 17 and 18, from an embryo operated upon as above, show a median cyclopean eye in which the pigment layer is wanting between the two components. The two cup cavities are also slightly reduced in size. This fusion of the eye rudiments in the median line has taken place in such a manner as to separate the cranial from the facial portions at the anterior end of the head, contrast Fig. 17 with a similar view of the normal head, Fig. 4. Figs. 19, 20 and 21 show a somewhat similar cyclopean eye. The operation was done at the stage shown in Fig. 2, and the embryo killed 15 days later. The sections show a common cup cavity, the retinal and pigment layers forming a continuous wall about the cavity. There are two 180 WTarrcn 11. Lewis. lenses and two pupils, however. The lenses are in contact. There are also two distinct optic nerves. The brain is reduced in size and the eye separates it from the mouth region. Figs. 22, 23 and 24 are from an embryo operated upon at a stage shown in Fig. 2 and killed 15 days later. Here is a single median cyclopean eye with one pupil and one lens and one cup cavity. A slight median notch on the anterior side of the optic cup indicates its origin from portions of two eye rudiments. The large optic cup shows in sections a very beautiful median eye with complete continuity of the layers of the rctinas of the two components about a single large cup cavity and a single lens. The two nasal pits are in contact and lie dorsal to the eye. The brain is somewhat reduced, and its anterior end separated widely from the mouth region by the medially placed eye. The explanation of the formation of these various abnormalities is in a way a comparatively simple one, if we assume that already in the early embryonic shield stage the various parts of the central nervous system and the eyes are, probably, already predetermined, and secondly that there is very little or no power of regeneration in this tissue. Numerous experiments on regeneration indicate very clearly that there is very little or no regeneration of the tissue (at least that of the central nervous system) extruded during the operation. Tlic repair, taking place after the operation, consists merely of a rapid closing together of the parts left behind, and thus, a healing of the wound occurs without regeneration of lost parts. This closing of the wound is accomplished in a few minutes, and rudiments are thus brought into contact that normally are quite widely separated, those of the two eyes, for example. The subsequent differentiation adjusts itself to the new relations of these rudiments with the resulting abnormal forms. Thus as one examines these developing embryos, from the very first time the eye rudiments are visible in the living specimen under the binocular microscope, they appear to have the same amount of fusion or loss of an eye that is clearly to be found in the same individual at later stages and at the time of hatching. So we can explain these cyclopean forms through a fusion of the rudiments of the two eyes immediately after the operation, even though a t this time no rudiments art! risible. Differentiation of the eye tissue c d e n t l y occurs sometime before it becomes visible by our crude microscopic methods. Thus cyclopia in man can be explained through the influence of esternal factors acting during early stages of development in such il iiianner as to prodlice a singlc eye ridiment, and we need not seek for a germinal explanation. I n these experiments on fish embryos, the eye rudiments wcre brought into contact and fused soon after the operation determining at this time the end result. There was not the formation of two eyes and then their subseqnent fusion into a single median eye. It seems likely that in man similar early fusion of the eye rudiments must take place to produce cyclopia. These experiments throw no light, of course, on the cause of the early defect in man, although Stockard's experiments indicate that chemical factors might be responsible for such defective or altered early development. The great similarity betwrwi these cyclopean forms and those produced by Stockard suggests that the lIgC1, may have i n some nianner prevented the growth of certain cells at the anterior end of the embryonic shield during the embryonic shield stage. It is possilde that the 31gC1, solution might have the same effect on eggs subjected to its influence during and just preceding the formaticxi of the embryonic shield.