GASTRULATION AND EARLIEST DEVELOPMENT 201 



the ectoderm (a) where the future anus might be expected, 

 if a similar state of affairs were totake place as in Anuians. 

 Immediately behind that shallow depression we find here 

 the same thickening of the ectoderm (*) as noted in f^ana 

 (cf figs. 1, 2, 3, plate II). Thus there is no fundamental 

 difference, on the contrary agreement in every respect with 

 what we found in Rana. 



Ndw in Rana we stated that the blastopore, after becoming 

 slit-like, continues to move backwards a small distance, 

 approaching the future anus, which manifests itself in 

 longitudinal sections in that the small lip which represents 

 the ventral blastopore border becomes a little shorter. 

 This now we see happening also in somewhat further 

 advanced stages of the axolotl-egg: in sections the ventral 

 lip gets shorter and soon, being here already small, it 

 disappears altogether. In the egg shown in fig. 41 b andc 

 (text) the medullary folds are on the point of fusing, except 

 at the fore and the rear end. The blastopore still appears 

 as a slit. The longitudinal section (fig. 7) shows that the 

 ventral blastopore lip has nearly disappeared: as a result 

 of the backward movement the rear end of the slit-like 

 blastopore has arrived at the spot where the anus must 

 break through! 



Particularly interesting is next the egg shown in fig. 41 

 d, where the medullary tube has just closed except at the 

 hindmost extremity where the anterior part of the slit-like 

 blastopore has just been overgrown by the medullary 

 folds. Whilst in Rana the whole blastopore is in this way 

 enclosed, in the axolotl the medullary folds leave an 

 opening over the rear end of the blastopore, which is 

 the anus (a). 



Only one egg in this stage was found by me amongst 

 my material. This was cut into longitudinal sections. 

 Morgan studied a similar egg in transverse sections. I repro- 

 duce here the outline of his exce'lentfigures which confirm 

 my views in every way. Fig. 42 a represents a section 

 through the medullary tube just in front of the blastopore. 

 Under it the anal diverticulum has been intersected. The 

 medullay folds just meet. Figs 42 b and c show the 

 blastopore in its anterior half, as is of course the case in many 

 succeeding sections The medullary folds meet over the blasto- 

 pore, the latter itself constituting the neurenteric canal. Figs 

 42 d and e are still further back, the medullary folds are 



