«50 



SPERM, OVA, AND PREGNANCY 



majority die before the fetal period is 

 reached (Blandau, 1952; also see Braden, 

 1959). 



A strikingly similar picture is presented 

 by delayed fertilization in the guinea pig. 

 The fertilizable life of the egg in this species 

 is approximately twice (20 hours) that of 

 the rat (Blandau and Young, 1939; Row- 

 lands, 1957). The first effects of over-ripe- 

 ness are seen in embryos from females in- 

 seminated approximately 8 hours after 

 ovulation. No normal development followed 

 inseminations more than 20 liours after ovu- 

 lation. As far as could be determined, the 

 principal effects of aging were either the 

 early death of the zygote in the pre-im- 

 plantation period or retardation in the rate 

 of growth in embryos which were capable 



of implanting. A moderate delay in fertiliza- 

 tion has been shown to lead to polyspermy 

 particularly in rats and rabbits (Austin and 

 Braden, 1953; Odor and Blandau, 1956). 



M. IMPLANTATION 



The blastula of the placental mammal is 

 called the blastocyst. In the fully developed 

 stage it is still enclosed in the zona pellu- 

 cida and shows the inner cell-mass attached 

 to the embryonic pole of the trophoblast. 

 During the early period of its existence the 

 blastocyst is spherical to somewhat oval in 

 shai)e and except for size appears remark- 

 ably similar from animal to animal (Fig. 

 14.i7). 



In most mammals, the blastocyst does not 

 come into firm contact with the maternal 



.•^ ^. 



Fig. 14.17. The similarity of the free uterine bhistoc-y.^t.s of various mammals: 1, 5!/2-day 

 human blastocyst (photograph courtesy, Hertig, A., and Rock, J.); 2, 6-day guinea pig blas- 

 tocyst; 3, 9-day monkey blastocyst (Heuser and Streeter, 1941); and 4, 9-day sheep blasto- 

 cyst (Boyd and Hamilton, 1952). 



