POLYPTERUS 



369 



teresting features. The cerebellum is large and well developed, but it 

 is hardly visible when the brain is viewed as a whole, for it is completely 

 involuted into the interior of the brain, there being not only a valvula 

 cerebelli as in the teleost but the hinder portion being similarly involuted 

 backwards into the fourth ventricle. So also with the hemisphere 

 region : the thalamencephalon is prolonged forwards, and the side walls 

 of the anterior portion, corresponding to the hemispheres, are much 



Pf 



FIG. 155. 



Larval stages of Polypterus. (From Graham Kerr's Embryology, after drawings by Budgett.) 

 A, stage 31; B, 33; C, 36; D, larva 30 mm. in length, c.o, Cement - organ ; E, eye; 

 e.g, external gill ; op, operculum ; p.f, pectoral fin. (A, B x n ; C x 8.) 



thickened and bulge inwards so as to be completely concealed in external 

 view. This modification clearly paves the way towards the condition in 

 the typical Teleost where the corresponding brain-material forms a solid 

 mass projecting inwards into the cavity of the brain. 



The Crossopterygians of to-day are the surviving remnants of a group 

 of extremely archaic vertebrates and consequently it is of much interest 

 to inquire how they develop. The young Polypterus passes through the 

 characteristic larval stages shown in Fig. 155. The most striking feature 



2 B 



