THE DEVELOPMENT OF THE VERTEBRATA. 



9 



The Development of the Vertebrata. — The ova of Verte- 

 brata have the same primary composition as those of other 

 animals, consisting of a germinal vesicle, containing one or 

 many germ,inal spots, 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 

 vietnhrane, and this may receive additional investments in the 

 form of layers of alhum,en, and of an outer, coriaceous, or cal- 

 cified sJieU. 



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.— Dlagrammattc section of the pregnant nterns of a decldnate placental mammal 

 (Homo): «, nterus; I, Fallopian tube; c, neck of the uterus; du, uterine decidua; rf*, 

 decidual serotina; dr, decidua r^jiexa; iB,e,' villi; cA, chorion; am amnion; nb^ 

 umbilical Teeicle; oZ, allantoie. 



Impregnation may take place, either subsequently to the 

 extrusion of the egg, when, of course, the whole development 

 of the young goes on outside the body of the oviparous 

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

 the latter case, the development of the egg 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 cicatricula, or 

 " tread," which is observable in the new-laid egg, is of this 

 nature. Or, the development of the 3'oung may be completed 



