386 EYCLESHYMER. [Vol, X. 



deren Rande der letzteren, indem sie einen geschlossenen Ring 

 bilden." 



The Zieglers ('92), in addition to thegastral mesoderm, describe 

 a layer extending around the entire periphery of the blastoderm: 

 *' In den nachsten Stadien schreitet die Bildung des peripheren 

 Mesoblastes rings um das ganze Blastoderm herum fort." 



The above observations all point unmistakably toward a 

 primitively unpaired condition of the mesoblast in Elasmo- 

 branchs. The formation of the axial mesoderm in some cases 

 preceding the differentiation of the anterior portion of the 

 peristomial is precisely what might be anticipated, since we 

 would expect to find the process much retarded in this region, 

 owing to the fact that through the great increase of yolk sub- 

 stance the most active area is far removed from that portion of 

 the germ-ring which must be considered as representing the 

 tail-end of the embryo. 



I have purposely omitted referring to the origin of the meso- 

 derm in Amphioxus, since Lwoff and E. B. Wilson have denied 

 the existence of the pole cells, which, according to Hatschek, 

 form the basis of the mesoderm. 



9. General Considerations. 



The evidence that the basis of the mesoblast in vertebrates 

 is a closed peristomial ring may be briefly summarized as fol- 

 lows : I have demonstrated this fact in Petromyzon and Am- 

 blystoma. Its existence in Elasmobranchs is beyond question. 

 Its presence in Teleosts has been repeatedly recorded. It 

 explains certain peculiarities observed in Sauropsida and Mam- 

 mals which have hitherto been unexplained, viz.y the formation 

 of a part of the mesoderm at the periphery of the blastoderm, 

 as observed in Tryonix and Clemmys by Mitsikuri ('91), and in 

 Sorex by Hubrecht ('90). 



I accordingly think the hypothesis well founded that the ver- 

 tebrate mesoblast must be considered as primarily peristomial. 

 The axial portion (axial mesoderm) in Amphibia has been car- 

 ried in by invagination ; while in forms containing a great 

 amount of yolk substance, and where overgrowth has replaced 

 invagination, the axial position is due to concrescence. 



