THE DEVELOPMENT OF THE VEETEBEATA. 3 



not communicate with the exterior — recalling, in the latter 

 case, the atrial cavities of Mullusca. 



In all Vertebrata, except Amphioxus, there is a single 

 valvular heart, and all possess a hepatic portal system ; the 

 blood of the alimentary canal never being wholly retui'ned 

 directly to the heart by the ordinary veins, but being more 

 or less completely collected into a trunk — the portal vein, 

 which ramifies through and supplies the liver. 



The Development of the Vertebrata. — The ova of Vertebrata 

 have the same primary composition as those of other animals, 

 consisting of a germinal vesicle, containing one or many 

 germinal spots, and included within a vitellus, iipon 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 invest- 

 ments in the form of layers of albumen, and of an outer, 

 coriaceous, or calcified 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. 



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 young may be completed 

 while the egg remains in the interior of the body of the 

 parent, but quite free and unconnected with it ; as in those 

 vertebrates which are termed ovoviviparous. Or, the young 

 may receive nourishment from its viviparous parent, before 

 birth, by the close apposition of certain vascular appendages 

 of its body to the walls of the cavity in which it undergoes 

 its development. 



The vascular appendages in question constitute the chief 



