THE BLASTOCYST O F THE F I S H E R ROBERT K. ENDERS AND O L I V E R P. P E A R S O N Fish and Wildlife Service, United States Department of the Interior, and Edward Martin Biological Laboratory, Swarthrnore College, Pennsylvania T W O FIGURES The fisher or pecan, Martes pennanti, breeds a short time after pal-turition, which may occur from late March to May, and carries the young for about 51 weeks (Laberee, '41; Hall, '42). While this is by no means the longest gestation period known in mammals, it is very long for so small an animal and for a mustelid. Recent work indicates that the long gestation periods observed in some weasels, Mustela frenata and 11. cicognani, and i n the marten, Martes americana, is correlated with delayed implantation (Wright, '42 ; llarshall and Enders, '42). I n these three animals as well as in the badgers Meles and Taxidea (Hamlett, ' 3 5 ) the fertilized ova develop to the blastocyst stage and remain in this condition for several months. Through the kindness of Dr. L. Butler of the Hudson's Bay Company, the reproductive tracts of three fishers were obtained. The ovaries of all three contained corpora lutea, so the uteri were sectioned serially and six blastocysts found. The material sectioned included tracts f rom animals killed in January, February and March. The uteri of the two animals killed in J a n u a r y and February contained unimplanted blastocysts, but none were found in the specimen killed March 18th, although one small corpus luteum was present in one ovary. The ovaries, follicles, and oviducts of this animal showed considerable enlargement indicating the approach of estrum. The specimen secured in January contained five corpora lutea arid three blastocysts. I n the February specimens there were three corpora lutea and three blastocysts. 285 286 R. I<. ENDERS B N D 0. P. PEARSON The blastocysts are similar in general appearance t o those of the marten, although slightly larger. They are of a very irregular shape, probably as a result of collapse of the blastocysts during histological preparation. The most conspicuous feature is the thick surrounding zona pellucida which averages 14.4 riiicra in thicliness. This is considerably thicker than in the marten, weasel or badger. The inner cell mass is less compact thaii in the iiiarteii, and is composed of a greater number Fig. 1 Blastocyst of fisher. Februauy. X 43. Fig. 2 Detail of ailother blastocyst showing part of inner cell mass. Fcliruarg. x 200. of cells. The inner cell mass of tm7o of the blastocysts from the January specimen contained 798 and 807 nuclei. This conipares with about 350 in the mai-ten, 150 in the weasel, and 50 in the badger. Preservation was not adequate t o allow an accurate nuclear count to be made on all of the blastocysts preserved in February, but a single count of 844 was possible. These later blastocysts appcar to be slightly more advanced than the January specimens, but there is no indication that implantation is imminent. BLASTOCYST OF T H E FISHEL: 287 Since blastocysts are found in January arid February following copulation the previous April or May, and since there is 110 evidence of delayed ovulation or fertilization in the mustelids, it seems probable that in the fisher unimplanted blastocysts exist from April or May until the following spring, n period of 9 or more months. L l T E R A T U R E CITED HALL,E. It. 1942 Gestation period in the fisher with recommendations for the animal’s protection in California. Calif. Fish and Game, vol. 28, pp. 143-147. HAMLETT, G. W. D. 1935 Delayed implantation and discontinuous developmeut in the mammals. Quart. Rev. Biol., vol. 10, pp. 432-447. LAREREE, E. E. 1941 Breeding and reproduction in f u r bearing animals. Fur Trade Journal of Canada, Toronto. MARSHALL, W. H., A N D R. K. ENDERS1912 The blastocyst of the marten. Anat. Rec., vol. 84: pp. 307-310. WRIGIIT,P. L. 1942 Delayed implantation in the long-tailed weasel (Mustela frenata), the short-tailed weasel (Mustela cicognani), and the marten (Martes americana). Anat. Rec., vol. 83, pp. 341-353.