BIOLOGY OF EGGS AND IMPLANTATION 



817 



after copulation. Shortly thereafter, the sec- 

 ond metaphase spindle is formed and re- 

 mains in position just below the surface of 

 the primary egg membrane. It remains in 

 this condition until the fertilizing sperma- 

 tozoon penetrates the egg. 



Similar observations on successive phases 

 of the first maturation division have now 

 l)een completed for the rat (Odor, 1955). In 

 over 1500 living and fixed eggs examined at 

 specific times before and after the onset of 

 heat it was observed that by the onset of 

 heat, the germinal vesicle has lost its mem- 

 iM'ane in most animals, and has been trans- 

 formed into a a dense chromatic mass which 

 then quickly moves towards the periphery 

 of the ooplasm. Between the 3rd and 4th 

 hours the chromosomes have arranged them- 

 selves in the metaphase plate. Abstriction of 

 the first polar body is usually completed 

 between the 6th and 7th hour, and position- 

 ing of the second metaphase spindle by the 

 8th hour. It is interesting that, even though 

 there was considerable variation in the 

 stages of maturation found in animals killed 

 at the same time after the onset of heat, 83 

 per cent of all the ova were in the same stage 

 of maturation or in a very closely related 

 phase. 



In all mammals studied, except the dog 

 and fox (Van der Stricht, 1923; Pearson and 

 Enders, 1943), the first maturation division 

 is completed within the ovarian follicle sev- 

 eral hours before it ruptures. 



There is evidence that a specific correla- 

 tion exists between the gonadotrophins and 

 the maturation phenomena within the oo- 

 cytes (Bellerby, 1929; Friedman, 1929; 

 Friedgood and Pincus, 1935). Apparently 

 the threshold of response of oocytes for mat- 

 uration is lower than is the threshold for 

 ovulation (Hinsey and Markee, 1933). Mor- 

 icard and Gothie (1953) injected small 

 quantities of chorionic gonadotrophin di- 

 rectly into the ovarian follicles of unmated 

 ral)bits and observed the formation of the 

 first metaphase spindles and the abstriction 

 of the first polar bodies. This was inter- 

 preted as showing the direct effect of pitui- 

 tary hormones in inducing meiosis. On the 

 basis of a study on oocytes recovered from 

 ral)bit ovaries Chang (1955b) concluded 

 that once the oocytes have attained the ve- 



sicular stage maturation can be readily in- 

 duced by a variety of experimental proce- 

 dures the most effective of which is the 

 subnormal temperature treatment of unfer- 

 tilized ova. According to his investigations 

 first polar body formation is not immedi- 

 ately dependent on gonadotrophic stimula- 

 tion. 



A number of investigators who have ex- 

 amined mammalian ova have commented on 

 the rapid disappearance of the first polar 

 body. Sobotta and Burckhard (1910) saw 

 the first polar body in only 2 of 100 recently 

 ovulated mouse ova. The infrequent pres- 

 ence of the first polar body in postovulatory 

 ova in which the second maturation spindle 

 was completed suggested that possibly the 

 first polar body was not always formed 

 (Sobotta, 1895) . Yet from a variety of stud- 

 ies on meiosis in fixed and living eggs, it may 

 be concluded that the abstriction of the first 

 polar body invariably occurs. In addition 

 it may not disappear as rapidly as some of 

 the older investigators believed. The first 

 and second polar bodies are visible in a 

 4-celled guinea ])ig embryo photographed by 

 Squier (1932). There has been considerable 

 speculation as to the method whereby the 

 first polar body disappears. Kirkham ( 1907) 

 suggested that the first polar body in the 

 mouse either was forced through the zona 

 pellucida or escaped from the perivitelline 

 space by its own ameboid movement. Simi- 

 lar theories have been held by Moricard 

 and Gothie ( 1953) for the rat, in which they 

 maintain that the polar body passes directly 

 through the zona pellucida. From the obser- 

 vations of Lams and Doorme (1908) in the 

 mouse. Mainland (1930) in the ferret, and 

 Odor (1955) in the rat, it is almost certain 

 that the first polar body undergoes rapid 

 fragmentation and cytolysis within the peri- 

 vitelline space so that only some finely 

 granular material remains. Ameboid move- 

 ment of the first body has never been docu- 

 mented in the thousands of living mammal- 

 ian eggs examined. 



E. THE OVULATED EGG 



The appearance of tubal ova from a single 

 animal varies considerably depending on the 

 lapse of time between ovulation and ex- 

 amination and the environmental fluids in 



