384 OUTLINES OF CHORDATE DEVELOPMENT 



A. THE GROWTH OF THE BLASTODERMIC VESICLE 



The enlargement of the vesicle results from the flattening 

 of the cells of the subzonal layer, as well as from their multipli- 

 cation, and the wall of the vesicle thus becomes very thin 

 (Fig. 149). Growth of the vesicle is always rapid, and often it 

 becomes very large. In the rabbit the ovoid vesicle reaches 

 dimensions of about 4.5X3.5 mm. by the seventh day of de- 



j 'cm 



trwph 



FIG. 149. Section through the fully formed blastodermic vesicle of the rabbit. 

 From Quain's Anatomy, after Van Beneden. fcm, Granular cells of the inner cell 

 mass; troph, trophoblast cells; zp, zona pellucida. 



velopment (third day within the uterus), when implantation 

 commences. In the mouse the spherical vesicle is much 

 smaller when implantation begins. In the Ungulates the vesi- 

 cle becomes elongated and tapered at each end, and very large; 

 in the sheep the twelve-days vesicle (Bonnet) reaches a length 

 of more than 20 cm., its diameter being only 1-2 mm. Ulti- 

 mately the Ungulate vesicle may extend through the entire 

 uterus, and may even be folded, so that a total bngth of 1 m. 

 may be reached. In all these cases the embryonic portion of 

 the early vesicle is limited to a small, almost microscopic mass 

 about in its middle. 



B. THE FORMATION OF THE EMBRYO AND THE EMBRYONIC 



LAYERS 



We may now return to a stage where the small blastodermic 

 vesicle is just established and consists of a single layer of 



