HALL : MESONEPHROS AND MULLEEIAN DUCT IN AMPHIBIA. 71 

 C. Comparison of the Mesoxephric Fundaments of Ambltstoma 



AND ICHTHYOPHIS. 



As the most important of my conclusions relates to the derivation of 

 the mesonephric fuinlaments, I have endeavored to make clear, by Fig- 

 ui-es E-H (p. 72), the similarity between the mode of formation of 

 the fundaments in Ichthyophis and Amblystoma. The comparison of 

 the two forms has importance not only from the fact that Ichthyophis 

 is a primitive Amphibian, but mainly from the fact that it is compara- 

 tively easy to homologize the processes in the formation of the mesone- 

 phric fundaments in that animal with those in elasmobranchs, a group 

 which has (in my opinion) a very primitive or generalized mesonephros. 



Figures E and F are based on Semon's ('92) Figures 9 and 10, the 

 interpretation of the extent of the various portions of the mesodermic 

 layers being, however, entirely my own. According to Semon's descrip- 

 tion, the development of a mesonephric fundament is briefly as follows. 

 In the earliest stage shown by him the cavity of the somite is already 

 separated from the body cavity by the fusion of somatoderm and 

 splanchnoderm (see Fig. E). The cavity of the somite then becomes 

 divided into a larger upper and a smaller lower chamber by the develop- 

 ment of a cross-partition (Fig. F), which thus forms the ventral wall 

 of the upper chamber and the dorsal wall of the lower. The partition 

 then splits into two layers, of which the dorsal becomes a part of the now 

 solid epimer (the lumen having become filled by proliferation from the 

 median wall), and the ventral furnishes a part of the wall of the lower 

 chamber. This chamber is the mesonephric blastula. Its region of con- 

 tact (Fig. F, nph'stm.) with the lateral mesoderm becomes broader ; 

 then by the ingrowth of connective tissue the contact is interrupted 

 except at two points, a lateral (Semon's " Contact a ") and a median 

 ("Contact b") region.^ The lateral region is destined to form the outer 

 tubule and nephrostome ; the inner the sexual cord. 



The direction and character of the shading will make clear what 

 portions 1 consider homologous in Figures E, F, G, and II, 



Figure G is based on the stage of Amblystoma represented by Figure 

 13 (Plate 2), Figure H on that represented by Figure 7 (Plate 1). 



Daring the process which in Ichthyophis (Fig. F) separates the me- 

 somer (" lower chamber ") from the epimer and convei'ts it into a 

 mesonephric blastula, its cavity remains as an actual lumen. In 



1 For the sake of simpheity this later differentiation has been omitted in 

 Figure F. 



