25 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 



Conclusion. 



In comparing together the Normentafeln of the three Dipnoi it will be seen at once how the close 

 affinity between Lepidosiren and Protopterus is brought out clearly in their developmental features. Their 

 whole course of development — segmentation, gastrulation, the modelling of the body of the embryo, the 

 characters of the larvae with their external gills and cement organ, and the internal details of development 

 so far as known — alike attest the naturalness of the group Dipneumona. 



Just as striking are the differences which mark off the developmental phenomena of Lepidosiren and 

 Protopterus from those of Ceratodus. In the latter the segmentation departs less from the equal holoblastic 

 type, the external features of gastrulation approach more near to those of the typical amphibian, and 

 striking differences are to be seen in the early larval stages. There is an absence of the tadpole shaped 

 stage so characteristic of Lepidosiren and Protopterus due to the fact that the yolk is in Ceratodus distributed 

 more equally along the length of the midgut rudiment. And with the absence of concentration of the yolk 

 towards the headward end of the midgut it is seen that the whole head region in Ceratodus proceeds more 

 rapidly in its development, while in Protopterus and Lepidosiren it is caused to lag behind in comparison. 



Apart from such general differences, the numerous differences in details of structure are enough to 

 make it piain that a deep cleft separates the monopneumona from the dipneumona although in my opinion 

 there still exists aniple reason for retaining them as subdivisions of a single group. 



