436 



VETERINARY PHYSIOLOGY 



conversion of some of the cells into neurons, and others 

 into neuroglia cells (fig. 171). 



The mesoblast on each side of this splits, and the outer part, 

 with the epiblast, goes to form the body wall (Somatopleur), 



FIG. 171. Transverse section of more FIG. 172. Longitudinal Section 

 advanced Blastoderm, to show through Embryo to show it sinking 



Epiblast, Mesoblast, and Hypo- down into ovum and the formation of 



blast, formation of Neural Groove the aninion, am. In the Mesoblast 



and splitting of the Mesoblast. round, all., the allantois, the blood 



vessels grow out to form the placenta. 



while the inner part with the hypoblast gets tucked in to pro- 

 duce the alimentary canal (Splanchnopleur) (fig. 171). 



The developing embryo sinks into the ovum, and, as a 

 result of this, the somatopleur folds over it and, uniting 



All. 



IL_ Am. 



FIG. 173. Schematic section through the pregnant uterus of the Mare to show 

 the large allantoic sac, All., filled with fluid surrounding the amniotic sac ; 

 Am., the fluid in which the foetus floats. 



above, encloses it in a sac the amniotie sac (fig. 172, am.), 

 which becomes distended with fluid the amniotie fluid, in 

 which the embryo floats during the later stages of its develop- 

 ment, and which acts as a most efficient protection against 

 external violence. 



