THE DEVELOPMENT OF THE YERTEBKATA. 



9 



Tlie Developmeiit of the Vertebrata. — The ova of Verte- 

 brata have the same primaiy composition as those of other 

 animals, consisting of a germinal vesicle^ containing one or 

 many germinal Sjyots^ and included within a vitellus, upon the 

 amount of which the very variable size of the vertebrate ovum 

 chiefly depends. The vitellus is surrounded by a vitelline 

 membrane^ and this may receive additional investments in the 

 form of layers of albumen^ and of an outer, coriaceous, or cal- 

 cified shell. 



The spermatozoa are always actively mobile, and, save in 

 some rare and exceptional cases, are developed in distinct 

 individuals from those which produce ova. 



Fig. 1. — ^Dia^-ammatic section of the pregnant nterns of a decidnate placental mamma! 

 (Homo): ii, uterus; I, Fallopian tube; c, neck of the uterus; du, uterine decidua; (7s, 

 decidua serotina; dr. decidua rejle^xa; z,z,' villi; c/i, chorion; am amnion; «&, 

 umbilical vesicle ; c/^, aUautois. 



Impregnation may take jolace, either subsequently to the 

 extrusion of the Gg^^ when, of course, the whole development 

 of the young goes on outside the body of the ompafous 

 parent ; or it may occur before the extrusion of the ^gg. In 

 the latter case, the development of the &gg in the interior of 

 the body may go no further than the formation of a patch of 

 primary tissue ; as in birds, where the so-called cicatricida^ or 

 " tread," which is observable in the new-laid ^gg., is of this 

 nature. Or, the development of the young maybe completed 



