88 GENERAL DEVELOPMENT. 



the medullary plate becomes established, as a linear streak 

 extending forwards from the blastopore over fully one half the 

 circumference of the embryo. The medullary plate first con- 

 tains a shallow median groove, but it is converted into the 

 medullary cord, not in the usual vertebrate fashion, but, as first 

 shewn by Calberla, in a manner much more closely resembling 

 the formation of the medullary cord in Teleostei. Along the 

 line of the median groove the epiblast becomes thickened and 

 forms a kind of keel projecting inwards towards the hypoblast 

 (fig. 39, tic]. This keel is the rudiment of the medullary cord. 

 It soon becomes more prominent, the median groove in it 

 disappears, and it becomes separated from the epiblast as a solid 

 cord (fig. 40, me). 



By this time the whole embryo has become more elongated, 

 and on the dorsal surface is placed a ridge formed by the 

 projection of the medullary cord. At the lip of the blasto- 

 pore the medullary cord is continuous with the hypoblast, thus 

 forming the rudiment of a neurenteric canal. 



Calberla gives a similar account of the formation of the neural canal to 

 that which he gives for the Teleostei (vide p. 72.) 



He states that the epiblast becomes divided into two layers, of which the 

 outer is involuted into the neural cord, a median slit in the involution 

 representing the neural groove. The eventual neural canal is stated to be 

 lined by the involuted cells. Scott (No. 87) fully confirms Calberla on this 

 point, and, although my own sections do not clearly shew an involution of 

 the outer layer of epiblast cells, the testimony of these two observers must 

 no doubt be accepted on this point. 



Shortly after the complete establishment of the neural cord 

 the elongation of the embryo proceeds with great rapidity. 

 The processes in this growth are shewn in fig. 41, A, B, and C. 

 The cephalic portion (A, 6-) first becomes distinct, forming an 

 anterior protuberance free from yolk. About the time it is 

 formed the mesoblastic plates begin to be divided into somites, 

 but the embryo is so opaque that this process can only be 

 studied in sections. Shortly afterwards an axial lumen appears 

 in the centre of the neural cord, in the same manner as in 

 Teleostei. 



The general elongation of the embryo continues rapidly, and, 

 as shewn in my figures, the anterior end is applied to the 



