286 



Veterinary Obstetrics 



line. From the deeper layers of this thickened, dense, primitive 

 streak, the cells grow out in lateral plates between the epiblast 

 and hypoblast, to constitute the permanent mesoblast. See 

 Fig- 25. 



^ 



Fig. 25. A transverse section across the hinder part of the em- 

 bryonal area of a rabbit embryo at the end of the seventh day, 

 the section passing through the primitive streak. X 80 Mar- 

 shall after KoUiker. 



E, Epiblast. H, Hypoblast. M, Mesoblast. 



PG, Primitive groove. PS, Primitive streak. 



Formation of the Embryo 



The embryo is formed in the embryonal, or germinal, area. A 

 longitudinal depression known as the neural groove is formed 

 immediately in front of the primitive streak, the long axes of the 

 two structures being parallel. The neural groove is bordered on 

 the sides by the neural folds, which grow upward from the epi- 

 blast and then approach each other to become united into a tube, 

 in which, eventually, the central nervous system is formed and 

 soon shows, in its anterior or head-end, the brain vesicles. 



By an infolding of the walls of the blastodermic vesicle about 

 the margins of the embryonic area, the latter becomes con- 

 stricted off from the rest of the vesicle, which then constitutes 

 the vitelline, or yolk sack. The yolk sack of the mammalian 

 embryo is small and of little consequence, as it contains no ap- 

 preciable amount of nutriment for the embryo, but simply a 

 quantity, of a presumably inert fluid. After this constriction 

 forms, dividing the embryo from the yolk sac, the dorsal surface 

 of the embryo grows much more rapidly than the ventral, which 

 causes it to bend ventralwards very rapidly, so that the head-end 

 is soon at right angles to the remainder of the embryo. The 

 head-end of the embryo, by becoming flexed, is bent downward 



