THE EARLY DEVELOPMENT OF THE FROG 113 



so early as to obscure certain features of gastrulation, but these 

 processes are both highly modified by the presence of a large 

 mass of inert yolk cells. The presence of this rather immobile 

 mass results in the formation of the embryo as on the surface 

 of a sphere instead of as a simple elongated embryo, and such 

 processes as the formation of enterocoelic or notochordal evagi- 

 nations of the gut seem more or less abbreviated when com- 

 pared with such a form as Amphioxus, where the embryonic 

 layers are not impeded in their foldings by any such restraining 

 influence as the yolk mass in the frog. Were we to assume 

 that the development of Amphioxus represents truly primitive 

 processes of development among the Chordates, we might 

 describe the course of early development in the frog by saying 

 that it is directed toward the accomplishment of the final 

 result, rather than the carrying through of a definite program, 

 so that the formation of the chief axial structures is effected 

 more or less independently of the formal processes of develop- 

 ment, as illustrated by Amphioxus. 



However, there is considerable doubt as to whether Amphi- 

 oxus does really represent, in these respects, conditions which 

 may be considered primitive for the Chor dates. Opinion 

 remains quite divided upon the subject of the relation between 

 developmental processes in Amphioxus and in such forms as 

 the Amphibia. Some would point out that excepting only the 

 Protochordates, all of the lower vertebrates have large eggs, 

 containing a considerable quantity of yolk, definitely localized 

 in one pole; that, indeed, the Mammals are the only Craniates 

 which have small eggs, with little yolk, comparable with those 

 of the Tunicates and Amphioxus. These would maintain that, 

 while we may say that the presence of the yolk in the egg has 

 modified the form of early development, we cannot call this 

 modified development typical for the Chordata. It is quite 

 likely that while the development of Amphixous is simpler and 

 more diagrammatic than that of any Craniate, we must after 

 all regard this as a secondary or acquired simplicity, and not the 

 simplicity of primitiveness. From the point of view of purely 

 comparative embryology, Amphioxus should be the first to be 



