22 J. GRAHAM KERR. 



notochoi-d, now circular in transverse section, develops a fine 

 cuticular membrane wliicli foreshadows the sheath, and in 

 longitudinal section its cells are seen to be becoming flat and 

 plate-like. 



In due course the notochord becomes separated off from 

 neighbouring structures by mesenchymatous tissue, partly 

 directly cut off the subchordal region of the hypoblast 

 (Text-fig. 4, h.in.), but for the most part arising by pro- 

 liferation from the inner surface of the mesoderm at about 

 the level of the nephric rudiment very much as in Selachians, 

 except that there is no obvious trace of a segmental arrange- 

 ment (Text-fig. 4, scl). I propose to postpone fui-ther con- 

 sideration of the mesenchyme till a later period. 



Origin of the Central Nervous System. — Already in 

 Stage 12, as has been mentioned (cf. PI. 3, fig. 14), the 

 epiblast has become somewhat thickened over the region of 

 the archenteron, the thickening affecting the lower layer 

 especially whose cells have become more regularly columnar. 

 By Stage 14, when there runs forward from the blastopore a 

 faint depression along the axis of the medullary plate, this 

 thickening has become more marked, and in addition the 

 deep layer of epiblast is becoming more than one-layered (cf. 

 figs. 15 and IG). The medullary plate thickening of the 

 epiblast, most marked along the mid-dorsal line, extends 

 outwards for a considerable distance, gradually thinning 

 away on either side. The axial portion of the medullary 

 plate rapidly increases in thickness, forming a deep wedge- 

 shaped keel, the rudiment of the neural cord. This medul- 

 lary keel develops from before backwards, and in some eggs 

 of Stage 14 it has already begun to be distinctly formed 

 anteriorly. By Stage 16 (cf. PI. 4, figs. 17 and 18), where the 

 medullary groove is well formed but widely open, the keel 

 has increased much in thickness, being about five cells thick 

 posteriorly, and thickening out anteriorly to about three 

 times as much. Just about the anterior limit of the archen- 

 teron the keel tapers off, first suddenly, then gradually, till 

 the ordinary two-layered condition of the general ectoderm 



