BIOLOGY OF EGGS AND IMPLANTATION 



837 



fragmentation of the vitellus has been ob- 

 tained for many different species (Hartman, 

 1924, Smith, 1925, in the opossum; Sobotta, 

 1895, Kirkliam, 1907, Long, 1912, Charlton, 

 1917, in the mouse; Chang and Fernandez- 

 Cano, 1958, in the hamster; Long and Evans, 

 1922, Mann, 1924, Blandau, 1943, 1952, in 

 the rat; Squier, 1932, Blandau and Young, 

 1939, in the guinea pig; Chang, 1950a, in the 

 ferret; Heape, 1905, Pincus, 1936, in the 

 rabbit; Dziuk, 1960, in the gilt; Hartman, 

 Lewis, Miller and Swett, 1931, in the cow; 

 and Allen, Pratt, Newell and Bland, 1930, 

 in man ) . 



With the possible exception of the rat, the 

 jiroblem of the ultimate fate of the degener- 

 ating ova has not been satisfactorily resolved 

 for any mammal. It is generally accepted 

 that as the unfertilized eggs undergo com- 

 l)lete fragmentation and dissolution they are 

 absorbed either in the oviducts or uterus 

 (Corner, 1928a; Pincus, 1936). Charlton 

 { 1917) suggested that final disintegration of 

 unfertilized ova in the mouse is effected by 

 means of phagocytic leukocytes. It is as- 

 sumed further that the unfertilized ova dis- 

 appear from the female reproductive tract 

 before the succeeding ovulation. However, 

 Hensen (1869) described the retention of 

 approximately 100 rabbit ova in a blocked 

 oviduct in which presumably the eggs had 

 accumulated from a number of ovulations. 



The unfertilized ova in the rat do not un- 

 dergo complete dissolution during the nor- 

 mal 4- to 5-day estrous cycle. The vitellus 

 fragments ordinarily into a number of units 

 of varying sizes and the eggs, with their 

 zonae intact, are eliminated near the end of 

 the succeeding heat period by being washed 

 out through the vagina (Blandau, 1943). 

 Attention has been directed to the freciuent 

 occurrence of abortive "cleavages" in the un- 

 fertilized tubal eggs of the ferret and rat 

 (Austin, 1949a; Chang, 1950a). This phe- 

 nomenon is more common in the prepubertal 

 rat treated with gonadotrophins than in the 

 adult animal. In the "cleaved" unfertilized 

 ova, the blastomeres and their nuclear con- 

 figurations may appear identical with those 

 of fertilized ova and can, indeed, be differ- 

 entiated only by the absence of the flagellum 

 of the fertilizing sperm. Most unfertilized 

 ova. however, fragment into a number of 



units of unequal size, each containing one or 

 more abortive nuclei. 



G. FORMATION OF THE SECOND POLAR BODY 



The penetration of the vitellus by a sper- 

 matozoon is not the only stimulus which will 

 induce the formation of the second polar 

 body. Yamane (1930) observed that if rab- 

 bit eggs are placed in solutions containing 

 rat or horse spermatozoa, or immersed in 

 pancreatic solutions, cytoplasmic masses 

 similar to the second polar body will be ab- 

 stricted. Similar "false polar bodies" or ex- 

 trusions of clear, chromatin-free masses were 

 produced when rabbit eggs were immersed 

 in various concentrations of trypsin ( Pincus 

 and Enzmann, 1936). Both the abstriction 

 of the second polar body and shrinkage of 

 the ooplasm may be induced in rabbit, rat, 

 and mouse eggs by a variety of other non- 

 specific stimuli such as ether, Nembutal, 

 nitrous oxide anesthesia, and "cold shock" 

 (Pincus and Enzmann, 1936; Thibault, 1949; 

 Austin and Braden, 1954b; Braden and Aus- 

 tin, 1954). By contrast, colchicine or "hot 

 shock" inhibits the emission of the second 

 polar body (Austin and Braden, 1954b). 

 Austin (1951b) described the formation of 

 the second polar body in rat eggs in which 

 spermatozoa were in the perivitelline space 

 but had not yet penetrated the vitelline 

 membrane. It is uncertain whether "activa- 

 tion" is caused by a substance released into 

 the perivitelline space by the spermatozoa, 

 or by the mechanical impact of the sper- 

 matozoa on the vitelline membrane. There 

 are relatively few data on the temporal re- 

 lationship between penetration of the vitel- 

 lus by the sperm and the abtrusion of the 

 second })olar body. Pincus and Enzmann 

 (1932) reported that in rabbit ova 45 min- 

 utes or more elapse between the time the 

 sperm enters the vitellus and the formation 

 of the second polar body is completed. For- 

 mation of the second polar body in vitro has 

 also been observed in mouse eggs that had 

 been penetrated by spermatozoa. The time 

 required for the complete process was over 2 

 hours (Lewis and Wright, 1935). Long 

 (1912) pointed out that second polar body 

 formation in the rat began within 5 minutes 

 to 2 or more hours after the spermatozoa 

 were added to tlie eggs in in vitro prepara- 



