GASTRULATION AND EARLIEST DEVELOPMENT 211 



of the stomodaeum (medullary tube) exceeds that of the soma, 

 so that the cardiac (neurenteric) pore overtakes the anus and 

 passes it. Just as in Annelids the position of the anus in 

 Vertebrates is terminal in regard to the soma proper, the tail 

 being an outgrowth of the dorsal side of the latter in a back- 

 ward direction. According to this conception the ventral 

 side of the tail belongs to the dorsal side of the soma. 

 In conformity with this the dorsal unpaired skinfold of the 

 fish- and amphibia-larvae is continued over the tip and the 

 underside of the tail as far as the anus. The mesoderm, 

 originating at the blastopore-border and evidently being 

 a product of the periporal growing zone, also takes a 

 considerable part in the tail-formation. 



Several authors have rightly emphasized the difference 

 between somatogenesis and what we may call with De 

 LanGE (1912) urogenesis, i. e. the formation of the tail. 

 From the foregoing results it appears that somatogenesis, 

 just as the somatogenesis in Annelids, is performed by the 

 perianal growing zone which gives rise to the fi.ture somatic 

 (not to the neural, which is that of the medullary plate) 

 ectoderm of the trunk which, as long as the medullary plate 

 is open, lies mainly ventrally and at the sides of the egg. 

 Simultaneously, however, with the gastrulation the periporal 

 growing zone is at work, which produces the backward 

 movement of the blastopore and the backward extension 

 of the originally crescentic rudiment of the medullary plate, 

 i. e. the rudiment of the medullary tube. Both growing 

 processes are combined with a third one, going on simul- 

 taneously: gastrulation, manifesting itself at the surface in 

 the contraction of the blastopore border. 



The urogenesis however sets in after two of these three 

 processes have finished, viz: gastrulation and the action of 

 the perianal or somatic growing zone. While in Anurans 

 both these processes stop nearly at the same time, in fishes, 

 as stated above, we frequently find hat somatogenesis 

 continues after gastrulation has been completed, so that 

 the anus eventually lies somewhere about halfway between 

 the yolk-cell-mass and the tip of the tail. The urogenesis 

 accordingly is exclusively the result of the periporal growing 

 zone which causes an elongation of the medullary tube 

 disproportional to the length of the soma. The difference 

 between somatogenesis and urogenesis herein finds an 

 explanation. 



