BIOLOGY OF EGGS AND IMPLANTATION 



841 



her of their nucleoH (Fig. 14.13, 5). During 

 the early hours of this period, the male pro- 

 nucleus grows at a more rapid rate than that 

 of the female, and this differential is main- 

 tained even until karyogamy. At the stage 

 of greatest development, the number of nu- 

 cleoli in the male pronucleus may have in- 

 creased to approximately 30 and that within 

 the female nucleus to 10. Near the end of 

 this interval, the pronuclei gradually ap- 

 proach one another. For some time after ac- 

 tual contact, the pronuclei retain their 

 identity and the female pronucleus may con- 

 siderably indent the larger male pronucleus 

 (Fig. 14.14, 2). Approximately one-half hour 

 before karyogamy begins, the nucleoli in 

 both in'onuclei disappear from view and 

 there is some shrinkage in the size of the 

 pronuclei (Fig. 14.14, 3). Even after the 

 complete disappearance of the nucleoli, the 

 nuclear membranes may still be intact. Soon, 

 however, they become irregular in outline 

 and disappear. Shortly before the first seg- 

 mentation division, an aggregation of the 

 pi'ophase chromosomes may be observed. 

 Within a brief period, the chromosomes are 

 arranged on the metaphase plate. After an 

 interval of 30 to 40 minutes, the chromo- 

 somes begin to divide and pass through the 

 anaphase and telophase stages (Fig. 14.14, 

 4 and 5). The first segmentation spindle 

 is observed most commonly between the 21st 

 and 23rd hours after the entrance of the 

 sperm. Even though Austin ( 1951c) followed 

 the formation of the segmentation spindle, 

 cleavage of the rat zygote did not occur in 

 vitro. 



It is often difficult to differentiate between 

 the male and female pronuclei in sectioned 

 material. Hence, their identification has not 

 been clearly established for most mammals. 

 The male pronucleus has been reported to be 

 larger in the cat (Hill and Tribe, 1924) , vole 

 ( Austin, 1957 1 , guinea pig ( Lams, 1913 ) , and 

 rat (Odor and Blandau, 1951 ; Austin, 1951c; 

 Austin and Braden, 1953) , and of approxi- 

 mately equal size in the mouse, guinea pig 

 (Lams and Doorme, 1908), bat (Van der 

 Stricht, 1910), cat (Van der Stricht, 1911), 

 and hamster (Boyd and Hamilton, 1952; 

 Austin, 1956b). 



Edwards and Sirlin (1956a, b, 1959) have 

 demonstrated that the male pronucleus 



within the fertilized mouse egg could be 

 identified by injecting adult males with 

 C^"*-labeIed adenine approximately 1 month 

 before mating. The male pronuclei showed 

 autoradiographs which could be related to 

 the labeled sperm particularly in di- and tri- 

 spermic eggs. Lin ( 1956) labeled unfertilized 

 mouse eggs with DL-methionine while they 

 were still within the follicles. Ovulation was 

 induced by gonadotrophins and the unferti- 

 lized eggs were transplanted to mated fe- 

 males where they were fertilized and subse- 

 quently delivered as normal young. 



The acridine orange-staining tcchni(iue 

 has been applied recently to living rat eggs 

 and the localization of the stain determined 

 by fluorescence microscopy (Austin and 

 Bishop, personal communication). The dis- 

 tribution of DNA may be determined by this 

 technique and the ]H-eliminary data give sup- 

 port to the earlier rejjorts of Dalcq and Pas- 

 tcels (1955) that duplication of DNA occurs 

 within the jironuclei. 



Information regarding the temiwral re- 

 lationship between the formation of the first 

 segmentation spindle and karyogamy is also 

 very meager. In the guinea pig (Rubaschkin, 

 1905; Lams, 1913), bat (Van der Stricht, 

 1910), and rat (Odor and Blandau, 1951), 

 the pronuclei have not completely fused by 

 the time the spindle is formed. Isolated 

 phases of this stage have been described also 

 for the mouse (Lams and Doorme, 1908), 

 rabbit (Gregory, 1930), and goat (Amoroso, 

 Griffiths and Hamilton, 1942). 



I. FATE OF THE CYTOPLASMIC COMPONENTS OF 

 THE FERTILIZING SPERM FLAGELLUM 



Observations on the extent to which the 

 flagellum of the fertilizing spermatozoon is 

 carried into the ooplasm of the mammalian 

 egg are contradictory and incomplete. The 

 majority of the reports deal with sectioned 

 material in which the identification of the 

 whole flagellum may be very difficult. Yet, 

 knowledge of the fate of the cytoplasmic 

 components of the sperm is essential to an 

 understanding of the role of the male gamete 

 and must be pursued further. 



In the mammals, the entire tail has been 

 reported to be lodged within the ooplasm in 

 the bat (Van der Stricht, 1923 ) ; mouse (Van 

 der Stricht, 1923; Gresson, 1948) ; guinea pig 



