

THE EARLY DEVELOPMENT OF THE FROG 115 



Amphioxus-like embryo, which has been modified by the ac- 

 cumulation of yolk and the precocious formation of the meso- 

 derm before confluence. It might easily be supposed that the 

 formation of the mesoderm in advance of the definite establish- 

 ment of the gut cavity would result in the loss of function and 

 disappearance of the enterocoels. In Amphioxus the gastral 

 and peristomial mesoderms have unlike origins, because the 

 mesoderm is not formed until after gastrulation, and conse- 

 quently, that formed from invagiiiated endoderm (gastral or 

 axial) is unlike that (peristomial or blastoporal) formed from 

 the "germ ring" or growth zone, around the posterior end or 

 blastoporal region of the embryo. 



While decisive evidence is perhaps lacking, and much may be 

 said on both sides, on the whole the evidence seems to favor the 

 first view; that the method of mesoderm formation in Amphi- 

 oxus is not wiiolly primitive, that primarily there is no distinction 

 between gastral and peristomial mesoderm, for all is first peristo- 

 mial or blastoporal, and that the mesoderm grooves of the frog 

 are not vestiges of enteroccelic evaginations, like those of Amphi- 

 oxus, but represent the primary mode of origin of the mesoderm 

 as a proliferation from the margin of the blastopore. This 

 would, of course, involve the conclusion that the ccelomic cavity 

 is not phylogenetically derived from the gut cavity among the 

 Chordata. In the frog the .ccelom has no relation with the 

 mesoderm folds, as it would have if these are vestigial enterocoelic 

 grooves, and it will be recalled that in Amphioxus only the cav- 

 ities of the more anterior somites are ever connected, even as 

 grooves, with the gut cavity. And yet in some of the tailed 

 Amphibia the ccelom is apparently sometimes directly con- 

 nected with the grooves in the mesoderm folds. 



Most of the differences in the arrangement of the mesoderm 

 rudiments in Amphioxus and the frog can be traced to this 

 difference: in the frog the mesoderm differentiates before gas- 

 trulation and confluence, in Amphioxus the mesoderm differ- 

 entiates after gastrulation and after confluence has begun. 

 Doubtless these unlike relations are largely the result of the 

 absence of the yolk mass in Amphioxus and the consequent 



