No. 2.] EARLY DEVELOPMENT OF AMBLVSTOMA. 389 



overgrowth. A comparison of successive stages during gastru- 

 lation shows a continual diminution of the segmentation cavity, 

 due to the extension of the gastral cavity, the part farthest 

 removed from the dorsal lip being the last to disappear. 



Fig. 3. ^G" 4' 



Fig. I. — Diagram showing the formation of the embryo as observed in Amphibia. 

 Fig. 2. — Ditto in Coregonus and most Teleosts. 



Fig. 3. — Ditto in certain Teleosts (Batrachus, Amiurus, Lophius) and Elasmobranchs. 

 Fig. 4. — Ditto in Chick after Rauber (modified). 

 The portion of the embryo (a.b.) formed by differentiation in situ is represented in black ; that 

 lonned through backward extension, or overgrowth (*.<:.) by dots, while the extent of concrescence 

 forming the primitive streak is represented by the heavy line p.s. 



The successive positions occupied by the germ-ring, or thickened blastodermic rim, are rep- 

 resented by t, 2, 3, 4, etc. 



These observations are the principal ones thus far brought 

 forth to support the theory of concrescence in Amphibia, and 

 to my mind they are far from convincing. 



The accompanying diagrams are introduced to illustrate the 

 modification arising in the formation of the embryo through a 



