COMPARATIVE VIEW OF BRAIN DEVELOPMENT. 223 



A thorough comparison of the corresponding stages of 

 development in the brain in the various Skulled Animals 

 (Craniota) is very instructive. If it is applied to the whole 

 series of skulled classes, the following extremely interest- 

 ing facts soon become evident : in the Cyclostomi (Myxi- 

 noides and Petromyzontcs), which, as we have seen, are 

 the lowest and earliest Skulled Animals, the whole brain 

 remains for life at a very low and primitive stage of 

 development, through which the embryos of the other 

 Skulled Animals pass very rapidly; the five original 

 sections of the brain are visible throughout life in an almost 

 unmodified form. But even in Fishes, an essential and 

 important transformation of the five bladders takes place ; 

 it is evidently from the brain of the Primitive Fishes 

 (Selachii ; Fig. 228), that, on the one side, the brain of the 

 other Fishes, and on the other, the brain of the Amphibians 

 and also of the higher Vertebrates, must be traced. In 

 Fishes and Amphibians (Fig. 229), the central part, the 

 mid-brain, and also the fifth, section, the after-brain, are 

 especially developed, while the first, second, and fourth 

 sections remain far behind. In the higher Vertebrates, the 

 exact reverse is the case, for in these the first and fourth 

 sections, the fore and hind brains, develop pre-eminently ; on 

 the other hand, the mid-brain remains very small, and the 

 after-brain is also much smaller. The greater part of the 

 " four-bulbs " is covered by the large brain (cerebrum) and 

 the after-brain by the small brain (cerebellum). Even 

 among the higher Vertebrates themselves, numerous grada- 

 tions occur in the structure of the brain. From the Am- 

 phibians upward, the brain, and with it the mental life, 



develops in two different directions, of which the one is 

 33 



