FOBMATION OF GEEMINAL LAYERS IN CHELONIA. 235 



niDg from the anterior wall of the blastopore, where they pass into the 

 epiblast, and proceeding forwards. Instead of being large, irregular, 

 full of yolk, as in the previous stages, they become columnar, lose 

 their yolk, arrange themselves in a definite layer several cells deep, 



and take on the characters of normal hypoblast This process 



is evidently an invagination comparable to that which takes place in 

 an Elasmobranch. It especially resembles the process described by 

 Scott and Osborne in the newt." Strahl gave his views first in an 

 article published in 1882 (No. 8), and again in a later writing (No. 

 13, p. 55). His views, as expressed in the latter, are briefly as fol- 

 lows: — Before the neurenteric canal is present the germinal disc 

 consists throughout only of ectoblast and entoblast, except in the 

 region of the primitive streak, which is oval or jjear-shaped, or 

 nearly triangular in form. In such a disc three processes, which 

 may be independent of one another, now take place. 



1. Under the primitive streak the entoblast is differentiated, so 

 far as it has not done so already. 



2. In the middle of the primitive streak the canalis neuren- 

 tericus is sunk, at first perpendicularly below and then horizontally 

 forward. 



3. In the region of the primitive streak the ectoblast differen- 

 tiates from the mesoblast. This differs in the regions before and 

 behind the neurenteric canal. In front of the canal the whole mass 

 is differentiated into the ectoblast and the mesoblast (i. e. mesoblast 

 according to his views : we would call it the chorda-entoblast). In 

 the region behind the canal, only the epidermal layer of the ectoblast 

 is differentiated, the differentiation of the remaining cells into the 

 structures for which they are destined : viz. the extreme end of the 

 medullary canal, of the chorda, &c., takes place at a much later date. 



As we stated before, we did not succeed in obtaining: the stasres 



