BIOLOGY OF EGGS AND IMPLANTATION 



845 



velopmental stages including the late blast- 

 ocyst. As mentioned earlier, spermatozoa 

 from the same insemination that are lying 

 free in the oviduct will have undergone 

 extensive cytolysis in less than 24 hours. 

 Finally, with the disappearance of the zona 

 pellucida at the time of implantation, the 

 accessory spermatozoa are cast forth into 

 the uterine lumen. Austin (1957) suggests 

 that the flagellum within the perivitelline 

 space of the vole egg may undergo dissolu- 

 tion in situ. 



The penetration of more than one sperm 

 into the ooplasm is a common phenomenon 

 in birds, rei:»tiles, urodeles, selachians, and 

 insects (Fankhauser and Moore, 1941). 

 Ordinarily, the additional sperm nuclei do 

 not interfere with the development of the 

 egg. 



Until recently, polyspermy in cutherian 

 eggs was considered to be relatively rare 

 (Austin and Braden, 1953). Nevertheless, 

 trinucleate eggs have been described in the 

 rat (Tafani, 1889; Kremer, 1924; Pesonen, 

 1949; Austin and Braden, 1953); cat (Van 

 der Stricht, 1911; Hill and Tribe, 1924); 

 ferret (Mainland, 1930) ; and rabbit (Amo- 

 roso and Parkes, 1947; Austin and Braden, 

 1953). 



According to Austin and Braden, the in- 

 cidence of polyspermy in the normally 

 mated rat is approximately 1.2 per cent; 

 in the rabbit 1.4 per cent. If mating is de- 

 layed until after ovulation or if rats are 

 subjected to hyperthermia, the figure rises 

 to as much as 8.8 per cent. The incidence of 

 polyspermy is no doubt influenced by a 

 variety of conditions including hereditary 

 variations within various strains (Odor and 

 Blandau, 1956; Braden, 1958a, b). Austin 

 and Braden (1953) concluded from their 

 work that polyspermy in rats gives rise to 

 triploidy in the embryo and that the poly- 

 spermic male pronuclei and the female pro- 

 nucleus contribute to the formation of the 

 first cleavage spindle. To the present, the 

 polyspermic rat embryos have been found to 

 develop to at least the 8-cell stage without 

 showing abnormality. Fischberg and Beatty 

 (1952a, b) have observed a normal-appear- 

 ing triploid mouse embryo at 9^2 days. It is 

 not known wdiether the triploid embryos 

 can survive to birth. More recently. Gates 



and Beatty (1954) have stated that delay 

 of fertilization by 5^/2 hours or more in the 

 mouse did not result in an increased number 

 of triploid embryos. 



K. STAGES OF DEVELOPMENT AND 

 LOCATION OF EGGS 



The zygotes of the eutherian mammals 

 are remarkably similar in their appearance 

 and rate of development through the vari- 

 ous stages of cleavage and formation of the 

 blastocyst. Cleavage consists of a succession 

 of mitotic divisions of the zygote at specific 

 time intervals after karyogamy. The par- 

 titioning of the zygote occurs with little or 

 no increase in the total amount of cyto- 

 plasm. Salient features of the mechanism 

 of cleavage in different vertebrate types 

 have been reviewed bv Bovd and Hamilton 

 (1952). 



Data on the rate of cleavage and trans- 

 port of fertilized ova through the oviduct 

 in different animals have accumulated much 

 more slowly than one would expect from the 

 availability of material. The most complete 

 information has been obtained for some of 

 the ungulates and laboratory rodents and 

 is presented in tabular form (Table 14.5) 

 from the summary of Hamilton and Laing 

 (1946). The rate of cleavage is an inherent 

 property of the zygote. Thus the cleavage 

 rates of different species of amblystoma 

 reared at the same temperature are signifi- 

 cantly different. Similarly, in the rabbit the 

 cleavage rate is consistently more rapid in 

 strains of larger-sized animals than in the 

 smaller-sized races (Castle and Gregory, 

 1929; Gregory and Castle, 1931). It is in- 

 teresting that, although the zygotes of the 

 larger-sized race divided more rapidly and 

 contained more cells, embryonic differentia- 

 tion occurred at the same rate in both races. 



Altering the environment of zygotes may 

 also effect the rate of cleavage. Thus the 

 early fertilized eggs of the rat, mouse, ham- 

 ster, and guinea jiig cleave only irregularly 

 if at all under tissue-culture conditions. If 

 various thio-amino acids are added to the 

 medium in which rabbit zygotes are being 

 cultured, cell division will proceed normally 

 and may even be accelerated (Pincus, 1937; 

 Pincus and Werthessen, 1938; Miller and 

 Reimann, 1940). 



