x. 6 EVOLUTION OF DIPNOI 279 



the male. This is one of several details (lack of ossification, un- 

 constricted notochord) which raise the suspicion that the living 

 Dipnoi have acquired their special characters by a process of 

 paedomorphosis or partial neoteny, that is to say, becoming sexually 

 mature in an early stage of morphogenesis. 



The development, again, shows similarity to that of amphibia and 

 dissimilarity from the other groups of fishes in that cleavage is total 

 and gastrulation takes place to form a yolk plug. There is therefore 

 no blastoderm or extra-embryonic yolk sac. However, the cells of the 

 vegetative pole contribute little to the shape of the embryo and indeed 

 may form a partially separate yolk sac. The larvae show distinct simi- 

 larity to those of amphibia, especially the larvae of Lepidosiren and 

 Protopterus, in which there is a sucker and external gills. 



The nervous system shows the same affinities as the rest of the 

 organization (Fig. 1 68). The forebrain is evaginated into a well-marked 

 pair of cerebral hemispheres. The roof of these, though not very 

 thick, is nervous and therefore is definitely of the inverted type, not 

 everted as in Actinopterygii. The optic lobes are little developed, the 

 mesencephalon being hardlv wider than the diencephalon. The cere- 

 bellum is small. A peculiar feature is the development of the inner ear 

 to form a special lobed saccus endolymphaticus, lying above the 

 medulla oblongata. The significance of this is not known, but it is 

 interesting that similar backward extensions of the ear are found in 

 amphibia. 



In many features, then, the Dipnoi differ from modern fishes and 

 resemble the amphibia, which evolved from the same stock. The early 

 crossopterygians probably competed somewhat precariously with 

 other fishes, as do the Dipnoi, and indeed many urodeles, today. It 

 was only after tens of millions of years of evolution during the Car- 

 boniferous and Permian times that numerous land animals arose, 

 in the form of the later amphibian and reptilian types (Fig. 211). 



