BOOK IX. 



EMBRYOLOGY. 



Embryology has for its object the study of the modifications which the ovum 

 undergoes, from the moment 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 

 dividt.d into three chapters. In the first, the transformations of tlie ovum which 

 produce the 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 deal 

 with the development of the foetus. 



CHAPTER I. 



The Ovum and its Early Embryonic Developments. 



Article I. — The Ovum. 



The ovimi of the domestic mammifers is a vesicle about ysW of ^'^ inch in 

 diameter (the germinal spot being from ^^Vo ^o iriVo of an inch), contained in 

 the Graafian follicle, in the midst of the cumidus pniliijera. 



It possesses : 1. An amorphous, transparent enveloping membrane, '01 mm. 

 thick, named the vitelline membrane, or zo7ia peJhicida. 2. A hazy vis(>id fluid, 

 holding in suspension a large number of dark-coloured granules and fat-globules : 

 this is the vilelhis, or yolk. 3. The germinal vesicle, a spherical transparent 

 nucleus lying to one side of the vesicle, and readily altered. The germinal spot^ 

 a kind of very brilliant nucleolus seen in the centre o^ the nucleus. 



According to Balbiani, there also exists in the ovum of all animals — from 

 insects up to Mammals — beside the germinative vesicle, a second nucleus — named 

 Balbiani's or the embryogenous vesicle — which plays a very important part in the 

 nutrition of the ovum and the phenomena succeeding fecundation. 



Article II. — First Embryonic Developments. 



These include three important events — the segmentation of the vitellus, the 

 formation of the blastoderm, and the appearance of the embryo. 



These phenomena have been particularly studied in the ovum of the Rabbit, 

 and it is to the researches of Van Beneden on this subject that science owes the 

 most important information on the first phenomena that occur after fecundation 

 in the Mammalia. 



1. Segmentation of the Yitellus. — This takes place immediately after 

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