GEOWTH IN LENGTH OF THE FROG EMBRYO. 225 



which it stood I admit that I now thiuk it iuaccurate. My 

 reasons for so thinking I will now proceed to give. 



Again, Morgan and Ume Tsuda say that we '^ apparently 

 at the outset have orientated the embryo wrongly, for they 

 state the segmentation cavity has a roof which ultimately 

 becomes the anterior wall of the gastrula ; for the anus which 

 marks the posterior end of the embryo appears at the opposite 

 side of the ovum, — that is, on the floor of the segmentation 

 cavity." 



I cannot understand their objection to this paragraph. 



Figs. 7 to 11 on PI. 24 are all placed with what we conceive 

 to be the dorsal surface (D.) directed towards the top of the 

 plate. 



As regards the ocular evidence of an invagination spoken of 

 by Jordan, it is a pity that more details are not given of the 

 observations. 



Is it possible to trace a cell, or a spot on the surface some 

 distance from the lip of the blastopore, to gradually approach 

 and fold over the edge and so disappear, or do only cells 

 actually on the edge seem to be affected by the process ? 



What is the cause of the invagination ? I can quite well 

 imagine that individual cells at the edge may, by multiplication 

 of their neighbours or themselves, be pushed over the edge, as 

 also might cells on the inner edge appear to be pushed out- 

 wards, if we could see that edge. The splitting theory still 

 seems to me to be the more probable for the commencement 

 of the archenteric cavity and its extension forwards. 



But, as I shall point out a little further on, there is un- 

 doubtedly an apparent overgrowth, and I think certainly an 

 actual overgrowth of the lower pole cells by the dorsal lip of the 

 blastopore, together with the lateral and ventral lips, as they are 

 formed at a later period. This process, however, should not, I 

 think, be compared with the process of gastrulation so called, 

 or formation of primitive archenteron, but should be considered 

 to be intimately connected with the growth in length of the 

 embryo. In other words, to follow the same line of argument 

 that I have used in the description of the rabbit embryo, the 



VOL. 37, PART 2. NEW SEE. p 



