xxxviii Introduction. 



permanent condition of dependency upon the cerebi'O-spinal nerves^ 

 and also from their position, they are often ranked with these 

 latter nerves rather than with the sympathetic system. 



Though the generative glands of all Vertebrata appear to be 

 hermaphrodite at certain periods of foetal life, they are, with 

 the exceptions of a few Fishes and Amphibians, differentiated as 

 either male or female organs, before the attainment of adult life. 

 In many Fishes, in Amphibia, and in all higher Vertebrata, the ova 

 are impregnated by sexual congress ; parthenogenesis and meta- 

 genesis are entirely unknown in this Sub-kingdom, and metamor- 

 phosis has only been observed amongst Amphibia and in a few 

 lower Fishes. In all Vertebrata, except certain osseous Fishes, the 

 ova are set free by dehiscence into the perivisceral cavity, whence 

 they are ordinarily taken up by the infundibuliform orifices of 

 bilateral oviducts, or as in a few Fishes left to find their way into 

 the circumambient water, through an azygos orifice in the abdo- 

 minal walls known as the 'porus genitalis.'' On the other hand, 

 vasa deferentia, directly continuous with the capsular envelope of 

 the testes, are found in all Vertebrata except the Amphioxus, 

 the Ci/clostomi, the Ganoidei, and the Eel. 



The yolk of the impregnated ovum sometimes undergoes entire, 

 sometimes only partial segmentation. The germinal membrane 

 very early divides itself into three layers, from the uppermost of 

 which the cerebro-spinal nervous centres and the cuticular systems 

 are evolved ; from the lowermost the epithelial structures of the di- 

 gestive tube, and its glands with the exception of the parotid ; and 

 from the intermediate layer all the other structures of the body, the 

 cutis vera, the nerves, muscles, bones, and the various vegetative 

 organs with the exceptions given. The first indication of the forma- 

 tion of the embryo is seen in the appearance of the 'primitive groove / 

 by the upgrowth of the walls of which, the cranio-spinal canal and 

 the cerebro-spinal nervous axis are both formed as demi-canals at 

 first, and as closed tubes ultimately, by the intermediate, and by the 

 upper layers of the germinal membrane severally. The chorda dorsalis 

 is developed along the infero-median line of these structures ; and 

 at a point corresponding to the level thus marked out, lamellar 

 prolongations are sent off downwards, which form the walls of the 

 inferior cavity of the vertebrate body, and are known as the laminae 

 ventrales. The intestine in all Vertebrata except Amphioxus (and 



