200 



Nachdruck verboten. 



The Derelopmeiit of the Vascular System of Ceratodus. 



By Wm. E. Kellicott. 

 With 2 (9) Figures. 



The Dipnoi remain conspicuous among the vertebrates for their 

 uncertain relationship, both to the other fishes and to the Amphibia. 

 Those who have investigated their palaentological history believe the 

 Dipnoi and Amphibia to have been derived independently fi'om 

 Crossopterygian stock and to have undergone more or less parallel 

 evolution. The fragmentary contributions from the study of their 

 embryology indicate their common origin and later separation. Com- 

 parative anatomy oifers no solution of this contradiction but rather 

 complicates the problem by disclosing many Elasmobranch resemblances. 

 The vascular system in particular exhibits well-marked Elasmobranch 

 and Amphibian characters. The investigation of the development of 

 this system was undertaken in order to determine as far as possible 

 whether these Elasmobranch resemblances are true or false and to 

 determine whether the Amphibian resemblances, so marked in the cleav- 

 age and gastrulation processes, would be found also through later 

 development. The complete and detailed results of this study will be 

 given elsewhere; and I wish here to call attention merely to a few 

 of the more important facts that have been determined. 



The heart, throughout its formation and development, is very 

 closely similar to that of the Amphibia, particularly the Urodela. The 

 lateral mesoblast in the cardiac region is wholly formed by delamin- 

 ation from the surface of the hypoblast. The muscular wall of the 

 heart is formed as a median unpaired structure by the infolding of 

 the splanchnic mesoblast, while the somatic mesoblast forms the peri- 

 cardial wall. The cardiac endothelium is derived from the hypoblast, 

 partly as a solid mass of cells continuing posteriorly from the thyroid 

 anläge, and partly, in the posterior region, as single cells or groups 

 of cells budded off from the ventral surface of the pharyngeal region. 

 In the most posterior region the endothelial cells come from the 

 region where the mesoblast is delaminating from the hypoblast and it 

 is impossible to say therefore whether they are more correctly described 



