778 



SCIENCE 



[N.S. Vol. XXV. No. 646 



tissue, the cavity of which disappears by 

 the proliferation of the cells, and this 

 rounded plate of trophoblast which now 

 constitutes the foetal portion of the true 

 placenta is brought into contact with the 

 epithelium of the dorsal portion of the 

 uterine cavity, giving rise to the true 

 placenta. 



Into the ventral surface of this tropho- 

 blastic plate extends the vascular meso- 

 derm to complete the allantoic portion of 

 the placenta. 



While these changes are taking place, 

 "the embryo has been rapid / developing, 

 nourished by a highly devciiped yolk-sac 

 placenta which ceases to be f auctional after 

 the completion of the true dorsal placenta. 



The Maturation of the Mouse Egg: W. B. 



CoE and W. B. Kibkham. 



The process of maturation and fertiliza- 

 tion in a mammalian egg was first de- 

 scribed in detail by Sobotta in 1895. His 

 work was with the mouse egg, and he re- 

 corded the formation of more than one 

 polar body in only one tenth of these eggs. 

 Gerlach, after a study of preparations 

 made as early as 1890, has recently revived 

 Tafani's theory that in the majority of 

 mouse eggs the second polar body is sup- 

 pressed. Gerlach 's conclusion is that when 

 a spermatozoon enters an egg some time 

 after it has formed the second polar 

 spindle, the second polar body fails to de- 

 velop, the spindle degenerating within the 

 egg. These observations differ not only 

 from almost all those previously made upon 

 other eggs, but also from the conclusions 

 since arrived at by Van der Stricht, 

 Heape and Rubaschkin, for the eggs of the 

 bat, rabbit and guinea-pig, respectively, 

 who all agree that two polar bodies are 

 regularly formed by every ripe egg. 



Careful study of numerous series of sec- 

 tions of eggs and ovaries of the white 



mouse have led to the following conclu- 

 sions : 



1. Two polar bodies are apparently 

 formed by every egg which ia capable of 

 development, the first polar body appear- 

 ing within the ovary, the second after 

 the entrance of the spermatozoon into the 

 egg. 



2. At the end of the spireme the number 

 of chromatin masses is between twelve and 

 twenty-four. 



3. Twelve masses of chromatin are cast 

 out with the first polar body, and a like 

 number remain in the egg. 



4. There is a sharp distinction in form 

 between the chromosomes of the first and 

 those of the second polar spindle. 



5. Every egg which we have seen in the 

 Fallopian tube before fertilization pos- 

 sessed a second polar spindle. 



6. The zona pellucida, which is quite dis- 

 tinct, may persist undiminished through 

 the early cleavage stages ; but in most cases 

 the first polar body escapes from it during 

 the process of ovulation, so that the ma- 

 jority of eggs after fertilization possess the 

 second polar body only. 



7. During the spring months ovulation 

 commonly occurs every twenty-one days, 

 independent of copulation. 



8. The number of univalent chromo- 

 somes in the second polar spindle is twenty- 

 four, of which the second polar body re- 

 ceives twelve, leaving an equal number to 

 form the egg nucleus. 



9. The second polar body is formed only 

 after the egg has been fertilized. 



10. The first and second polar bodies dif- 

 fer in size, shape, and especially in chro- 

 matin content, so that they are easily dis- 

 tinguishable. 



11. At least the greater part, if not the 

 entire sperm tail enters the egg at the 

 time of fertilization. 



12. Since the mouse egg in every case 

 which we have observed forms two polar 



