372 EYCLESHYMER. [Vol. X. 



While the majority of observers have denied the existence 

 of a continuous layer of mesoblast at the dorsal lip of the blas- 

 topore, there are certain observations scattered through the 

 literature which show that its presence in some forms, at least, 

 is an undisputed fact. Gotte ('75), in Bombinator, finds all 

 three layers existing at the dorsal lip when the first indications 

 of gastrulation appear. 



Schultze ('88) says concerning the existence of the mesoblast 

 in this region in Rana, that " Schon auf dem Stadium der Be- 

 ginnenden Gastrulation in der dorsalen Urdarmrand drei 

 Keimblatter existieren." 



Houssay ('90) has found the same to be true in the Axolotl, 

 and says : " L'etude de I'Axolotl confirme done tout ce qui a 

 et^ dit par Gotte et Schultze, a propos des Anoures." 



Perenyi ('89) in Bombinator found the three layers of the 

 epiblast to be folded in at the lip of the blastopore, the outer- 

 most becoming invaginated hypoblast, while the others gave 

 rise to mesoblast. 



Marshall ('93) states that in Rana, " At the lip of the blasto- 

 pore, round its entire circumference, the three germinal layers, 

 epiblast, mesoblast, and hypoblast, are indistinguishably fused 

 together." 



The chorda. — Concerning the origin of the Chorda dorsalis 

 in Amphibia, no less than a score of investigators have recorded 

 their opinions. 



Among those who hold that the chorda is of entodermic 

 origin are : Scott and Osborn (-79), Bambeke ('80), Hertwig 

 ('82), Orr ('88), Houssay and Bataillon ('88), Robinson and 

 Assheton ('9i), and Jordan ('93). 



G5tte ('75) and Schultze ('88) dissent from the above, and 

 maintain that it arises from the mesoderm. 



When the infolding has progressed to the extent shown in 

 PI. XXI, Fig. 4, the outline of the chorda resembles the sec- 

 tion of a cone. It consists of a single layer of invaginated 

 hypoblast forming the roof of the gastral cavity {g.c). As 

 gastrulation proceeds, this layer of hypoblast extends and 

 assumes an obovate form, consisting throughout of a single 

 layer of columnar cells which on either side pass over into the 



