GASTRULATION AND EARLIEST DEVELOPMENT 



207 



be necessary to test the accuracy of the conclusions reached 

 by the application of the above principles. They are as follows. 

 Inter/erence of both with the gastrulation. — The ectoderm 

 which afterwards has to invest the whole soma — dorsally 

 also — lies in a stage like that of figs. 40 a and b (text) 

 principally at the ventral and the lateral sides and oniy 

 afterwards, by the closing of the medullary tube, extends 

 over the dorsal side as well. The production of this somatic 

 ectoderm must now evidently be performed by the perianal 

 growing zone, as is the case in Annelids. Thus in the 

 neighbourhood of the future anus, a short distance behind 

 the ventral blastopoie lip, mitoses may be expected to be 

 most frequent. When, however, the blastopore has closed 

 (figs 40, 41 a), the rearward extension of this ventral 

 ectoderm comes to an end. If now the perianal growing zone 

 continues to be active, a ring-shaped thickening of the ecto- 

 derm round the anal 

 pit will result. This 

 having been observed, 

 it appears to me that 

 it is here we possibly 

 have to look for the 

 explanation of the 

 ectodermal thickening 

 which in the figs. 1, 

 2 and 3 (plate) are 

 seen developing in an 

 increasing degree just 

 under the anal pit (*), 

 and which, as parame- 

 dian sections teach us, 

 reach forward also at 

 the left and the right 

 of it. In the axolotl, 

 where the extension of 

 the ventral ectoderm 

 is so slight, this ectodermal thickening also, though present, 

 is yet of very little importance (6 *). The activity of the 

 perianal growing-zone seems to die down soon afterwards 

 and the ectodermal thickening in the ensuing stages gradually 

 disappears again. Somatogenesis has closed simultiineously 

 with gastrulation. Were it to continue also after the end of 

 gastrulation the anus would eventually lie somewhere 



C 



prbstomium. 



Fig. 43. Larva of the sturgeon, after 

 KuFFFER from Hertwig's Handbuch, 

 Bd. I, 2, p. 24. 



1 limit of the gastrulation, 2 limit 

 of the somatogenesis, 3 limit of the 

 urogenesis. 



Beneath : Diagram of the interference 

 of the gastrulation (a) with the action 

 of the perianal (b) and the periporal 

 (c; growing zones. 



