426 MAMMALIA. 



ovary, burst, and release the contained ovum and a quantity 

 of the liquid which surrounds it. 



The ovum is a spheroidal cell about xi^ to -j-J-j- of an inch 

 in diameter, and is surrounded by two membranes — the outer, 

 the zona radiata, and an inner thin membrane. "Within this 

 thin membrane, shown to exist by E. v. Beneden, is the vitellus 

 or yolk, filled with fatty and albuminous granules. In this 

 vitellus is the germinal vesicle (nucleus), with one or two 

 nucleoli. The ova burst from the ovary, when the latter is 

 clasped by the funnel-shaped end of the oviduct. They then 

 pass down the tube, being fertilised in its upper extremity by 

 the entering of a single spermatozoon, and soon commence 

 to undergo the process of segmentation. Segmentation is 

 complete, the whole ovum being affected. This is spoken of 

 as complete or holohlastic segmentation. The bird's egg only 

 undergoes partial or merohlastic segmentation, owing to the 

 quantity of yolk in the ovum. Segmentation lasts seventy- 

 two hours in the rabbit, according to Balfour — this stage 

 varying in different animals to a considerable extent. When 

 complete the ovum consists of an outer layer of cubical cells 

 surrounding an inner layer of granular polygonal cells, except 

 at one spot where the inner granular cells are exposed, the so- 

 called lilastopore (fig. 202, BT). When segmented the ovum 

 passes into the uterus and becomes attached to it. Here the 

 blastopore closes up. The vesicle then produced is formed 

 by a cleavage between the outer and inner layers, which 

 gradually increases until the inner granular mass is only to he 

 seen at the point of the now closed blastopore {Hij 2). During 

 this change the ovum enlarges rapidly, so that there is formed 

 an extensive cavity in it, the inner granular mass now only 

 forming a small patch against the outer layer at one spot. 

 This cavity is the lilastodermic vesicle (Bl.v). Surrounding it 

 is the zona radiata and an albuminous layer (Alb.E). As 

 the vesicle grows the inner mass of cells spreads out and forms 

 an irregular layer beneath, except at its original position, in 



