114 OUTLINES OF CHORDATE DEVELOPMENT 



considered; from the phyletic standpoint it should be consid- 

 ered after the more typically Chordate frog not that the frog 

 is embryologically typical of all Chordates, merely that it rep- 

 resents that condition more truthfully than Amphioxus. On 

 the other side, some would hold that the embryological sim- 

 plicity of Amphioxus is that of true primitiveness, that Am- 

 phibian development is secondarily modified, and phyletically 

 a modification of that of the Protochordates, interpretable 

 only through the latter and not vice versa, and that many of the 

 differences are the result of the accumulation of yolk in the 

 Amphibia. We may mention from these two points of view 

 only the development of the mesoderm as one of the central 

 points. 



Except in Amphioxus the Chordate embryo remains two 

 layered or didermic only a very short time, on account of the 

 very early development of the mesoderm in all other forms. 

 In the frog the mesoderm cells are found, soon after the endo- 

 derm begins to be formed, first all around the margin of the 



d? / _ ^ . ,^____^^.^__^C^L^^ 



blastopore forming an important part of the germ ring; that is, 

 the mesoderm is first all blastoporal or peristomial. Then con- 

 fluence begins and the lateral portions of the germ ring are car- 

 ried to the mid-dorsal region and poured into the posteriorly 

 elongating embryo, where they form the mesoderm bands; the 

 mesoderm of the germ ring thus becomes axial in position and 

 is known then as gastral mesoderm. The gastral mesoderm is 

 here derived from the peristomial through a change in position 

 due to confluence, and no essential distinction between the two 

 is to be drawn. Only in the posterior region of the frog embryo 

 immediately in front of the blastopore, are there traces of 

 evagination in connection with the formation of the mesoderm 

 in the form of a pair of slight grooves or slits. These may be 

 considered as due merely to the rapid, unilateral and localized 

 proliferation of the cells around the blastopore, such as often 

 leads to a grooving of the surface, and as having nothing to 

 do with the mesoderm folds and enterocoelic evaginations of 

 Amphioxus. Or, on the other hand, these grooves may be 

 regarded as vestiges of the enterocoelic grooves of a primitive 



