70 THE CHANGES ¥ROM STAGE A TO F. 



structures? or do they receive nuclei from the adjacent ecto- 

 derm and endoderm? It must be remembered that the first 

 somite is separated from the anterior end of the mesoblastic 

 bands before the latter have reached the level of the middle of 

 the blastopore, so that we are now only concerned with the old 

 embryos of Stage a. After prolonged and careful study of 

 a large number of embryos, I have come to the conclusion that 

 this is a point which it is impossible to settle with certainty by 

 a study of preserved specimens. Still there is a certain amount 

 of evidence, which, on the whole, tends to show that the inter- 

 mediate nuclei at the lips of the blastopore do contribute to 

 the bands. The evidence depends upon the appearances pre- 

 sented by these nuclei at the lips of the blastopore. These 

 are constantly met with in a state of division, i. e. present- 

 ing the figures which are characteristic of dividing nuclei. 

 This is shown clearly in many of my figures, e. g. (Plate V, 

 fig. 26 d ; and Plate VI, fig. 3). Further, immediately within 

 these intermediate nuclei, and lying between the ectoderm and 

 endoderm, there are often to be seen nuclei, which may fairly 

 be supposed to have been derived by division from the inter- 

 mediate nuclei. These are present in sections through the 

 young mesoblastic bands (Plate V, fig. 26 b), and also in 

 sections in front of the latter (Plate VI, fig. 3, close to d. n.) ; 

 in the former case indicating that the nuclei in question are 

 reinforcing the nuclei of the mesoblastic bands ; in the latter 

 that they are laying down the same structures. 



2. With regard to the second question, I can only state that 

 the completed somites attain their final position by an altera- 

 tion in the relative position of the structures in the anterior 

 region of the embryo. 



Plate VI, figs. 4, 5 a — -/, are illustrations of the structure 

 of the embryo at the close of Stage b (Plate II, fig. 25). Fig. 4 

 is in front of the mouth, and is through the first pair of somites, 

 now pre-oral. Fig. 5 a is through the mouth and second pair 

 of somites ; fig. 5 b between the mouth and anus, through the 

 region in which the blastopore lips have come together and 

 fused ; fig. 5 c is through the anus ; fig. 5 d, e, f, behind the 



