38 AUTHUR. DENDY. 



more or less, however, in the mid-dorsal and mid-ventral lines 

 towards the anterior extremity. 



The Primitive Streak and Neurenteric Canal. — 

 The primitive streak, or at any rate the posterior mass of 

 mesoblast derived therefrom, forms a dense mass of cells 

 behind the neurenteric opening, somewhat triangular in 

 transverse section, covered by flattened epiblast, and arched 

 over by the true amnion (fig. 71). The blastopore has dis- 

 appeared from view externally, being included in the closure of 

 the hinder part of the medullary groove ; but the neurenteric 

 passage still leads downwards through the floor of the 

 medullary canal, and places the cavity of the latter in free 

 communication with the cavity of the unenclosed portion of 

 the alimentary canal below (fig. 70). 



The Notochord. — The notochord can now be traced 

 forwards to a point just in front of the anterior extremity 

 of the alimentary canal, where it lies above the latter in the 

 angle between the mid- and fore-brain. Posteriorly it can, of 

 course, be traced back to the neurenteric aperture, with the 

 mart^in of which it is continuous. In the trunk region it is now 

 widely separated from the underlying hypoblast (figs. 57 — 69). 



Mesoblastic Somites, Coelom.ic Cavities. — The meso- 

 blast on each side of the spinal cord has become segmented 

 into fourteen mesoblastic somites, the first of which lies just 

 behind the auditory pit, and the last some distance in front of the 

 neurenteric aperture. These somites are cubical blocks, whose 

 cells have the usual columnar form and radial arrangement 

 (figs. 65 — 68, M. S.). Anteriorly they are connected with the 

 peritoneal epithelium by the intermediate cell mass (figs. 55, 

 67 68, /. C. M.), beyond which the somatopleure and the 

 splanchnopleure diverge, leaving between them only a narrow 

 coelomic space in the body of the embryo, which is continuous 

 with the very large pleuro-peritoneal space outside (fig. 68). 

 Posteriorly, about the region of the twelfth mesoblastic somite 

 (fio-. 69), the coelomic split can still be traced into the somite 

 itself, the latter having not yet completely separated off from 

 the lateral plates of mesoblast. 



