CHAPTEK XY. 



SEGMENTATION OF THE VITELLTJS AND FORMATION OF THE MEM- 

 BRANES AND PLACENTA. 



Deformation and gyration of the vitellus Polar globule Vitelline nucleus 

 Segmentation of the vitellus Primitive trace of the embryon Blastoder- 

 mic layers Formation of the membranes Formation of the amnion 

 Amniotic fluid Formation of the umbilical vesicle Formation of the al- 

 lantois and the permanent chorion Umbilical cord Membranae deciduae 

 Development and structure of the placenta Structure of the fully-devel- 

 oped placenta Blood-vessels of the placenta. 



As we have seen in the preceding chapter, it is probable 

 that the ovum is fecundated, either just as it enters the Fal- 

 lopian tube or in the dilated portion near the ovary. As it 

 passes down the tube, whether it be or be not fecundated, it 

 becomes covered with an albuminous layer. This layer prob- 

 ably serves to protect the fecundated ovum, and, when the 

 spermatozoids do not penetrate the vitelline membrane near 

 the ovary, presents an obstacle to their passage. Shortly 

 after fecundation, the germinal vesicle disappears ; but this 

 occurs in ova that have not been fecundated. Soon after 

 ovulation, also, the vitellus gradually withdraws itself from 

 certain portions of the vitelline membrane, or becomes de- 

 formed, and then often rotates upon itself ; a phenomenon 

 which has long been observed in the ova of some of the lowest 

 orders of animals, but which was seen by Bischoff in the ova 

 of rabbits, and was thought by him to be due to the move- 

 ments of cilia upon the surface of the vitellus. 1 The pres- 

 ence of cilia in this situation, however, has not been con- 



1 BISCHOFF, Traite du developpement de Vhomme et des mammiferes. Ency- 

 clopedic anatomique, Paris, 1843, tome viii., p. 598. 



