BOOK IX. 



Embryology. 



EMBRTOiiOGY has for its study the modifications to whioli the ovuliun is 

 submitted, from the moment when it is fecundated until it is transformed 

 into a new being capable of living in the external world. 



The points of this subject belonging to the domain of anatomy, will be 

 divided into three chapters. In the iirst, the transformations of the ovulum 

 which produce the ovum and embryo will be examined. In the second, the 

 various portions of the ovum — the annexes of the foetus — will be studied ; 

 and the third will be occupied with the development of the foetus. 



CHAPTEE I. 



THE OVULUM AND ITS MODIFICATIONS AFTER FECUNDATION. 



Article I. — The Ovultjm. 



The ovulum of the domestic mammifers is a vesicle about y^^j-j- of an 

 inch in diameter (the germinal spot being from ^^Vff *o tttVit ^^ ^^ inch), 

 contained in the ovisac, in the midst of the cumulus proligera. It possesses 

 all the elements of a complete cell, comprising : 1, An amorphous, trans- 

 parent enveloping membrane, named the vitelline membrane, or zona pellucida ; 

 2, A hazy viscous fluid, holding in suspension a large number of dark 

 granulations and fat globules : this is the vitellus or yelh ; 3, The ger- 

 minative vesicle, a spherical, transparent nucleus lying to one side of the 

 vesicle, and readily altered ; 4, The germinal spot, a kind of very brilliant 

 nucleolus seen in the centre of the nucleus. According to Balbiani, there 

 also exists in the ovulum of all animals, from insects up to mammals, 

 beside the germinative vesicle, a second nucleus — named Balbiani's or the 

 emhryogenous vesicle — which plays a very important part in the nutrition of 

 the ovulum and the phenomena succeeding fecundation. 



Aeticle II. — Modifications in the Ovultjm until the Appearance of 



THE Embryo. 



These include the segmentation of the vitellus, the formation of the 

 blastoderm, and the appearance of the embryo. 



1. Segmentation of the Vitellus. — Several times spermatozoa have 

 been seen in the vitelline zone of the fecundated ovulum. This fact is merely 

 alluded to, as we have to speak of the consequences of fecundation, and not 



