ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



195 



remain in that stage for some months, before continuing its development. 

 Although many of the walruses taken in March and April (and many in May and 

 June, as well) had a large, well-developed corpus luteum, most had no visible 

 embryo in the uterus (Brooks 1954; Burns 1965; Krylov 1969). These animals 

 have been assumed to be pregnant, with the embryo still in the blastocyst stage. I 

 investigated this in two females, taken on 14 and 30 May. By saline perfusion of 

 the lumina of their uterine horns, I was able to isolate the blastocysts, which 

 were 0.4 and 0.9 mm in diameter, respectively (Fig. 117a). 



Time of Implantation 



The rate of growth of the mammalian conceptus is inversely proportional to 

 the length of the gestation period, from nidation (implantation) to birth. Thus, 

 the size of the fetus at any instant in its development, relative to the size at birth, 

 is indicative of the amount of gestation completed and to be completed. In many 

 species of mammals, the length of the embryo at the time of its attachment to the 

 uterine wall is about 3 to 4% of the crown-rump length at birth (Cloete 1939; 

 Winters et al. 1942; Patten 1953), at which size it is in the neurula stage (Evans 

 and Sack 1973). By analogy, the length of the embryonic Pacific walrus at 

 implantation would be about 3 or 4 cm, and the time of implantation would be 

 in late June to late July. My findings and those reported by Belopol'skii (1939), 

 Collins (1940), Brooks (1954), Krylov (1966Z?, 1969), and J. J. Burns 

 (unpublished data), lend support to that prediction (Table 32). 



Embryos large enough to be distinguished without magnification were 

 identified first in 2 of 70 females taken on 22 and 25 May with a well -developed 

 corpus luteum of pregnancy. Each embryo was an elliptical blastocyst, 6 x 

 4 mm in diameter, in which the blastoderm (about 3 mm long) was in the 

 primitive streak stage of development (Fig. 117i>). Two other blastocysts in this 

 condition were recovered from animals taken on 2 June and one other from a 

 female taken on 7 June. Although there was no gross evidence that these elliptical 

 bodies were attached to the endometrium, I assume because of their size and 

 shape that they were in an initial stage of nidation (cf. Mayer 1963). 



Table 32. Frequency of occurrence per month of unattached blastocysts and 

 of implanted embryos and fetuses in Pacific walrus females having a corpus 

 luteum of pregnancy ."^ 



Unattached blastocyst 



Embryo 

 implanted 



Month 



Not visible 



Visible 



February 

 March 



1 

 9 

 11 



68 

 38 

 0 

 0 

 0 



0 

 0 

 0 

 2 

 3 

 1 

 1 

 0 



0 

 0 

 0 

 0 

 7 

 9 

 23 

 78 



April 

 May 

 June 

 July 



August 

 September 



^Data from Krylov (1966^?, 1969) and J. J. Burns and F. H. Fay (unpublished data). 



