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The blastocyst of the fisher.

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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.
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