BY WILLIAM A. HASWELL, M.A., D.SC. 589 



the mesoblast cells of the lateral plate. Without examining series 

 of sections of a somewhat earlier stage than this it would be im- 

 possible to say positively that the notochord does not here arise 

 from the hypoblast, but from the mesoblastic portion of the lower 

 layer after the hypoblast has become separated from it as a definite 

 layer of flattened cells : yet the similarity in character between the 

 cells constituting this early rudiment of the notochord, together 

 with the special character of the hypoblast cells and the absence 

 of any transition forms between the two, would seem to strongly 

 favour such a supposition. 



There is no demarcation in the series of sections between 

 the medullary groove and the primitive groove — the one 

 passing insensibly into the other (plate XIV. fig. 19.). Where the 

 axial groove becomes shallower behind the head-swelling the 

 axial part of the mesoblast becomes continuous with the epiblast 

 at the bottom of the groove and with the hypoblast below ; 

 and this coalescence of the three layers alone mai-ks the passage 

 from the embryonic region to the region of the primitive streak 

 (fig. 20.). At its posterior end the primitive streak is elevated in 

 the form of a ridge along the middle of the anterior part of which 

 runs the primitive groove. In this region there are still only two 

 layers — upper and lower. 



In the next stage examined (plate IX. fig. 4) (in which, however, 

 incubation had only gone on for 69 hours) the blastoderm was 

 about five centimetres in diameter, the head-fold had become con- 

 siderably further advanced, the medullary groove had become 

 greatly increased in length, and the medullary folds much more 

 prominent, though they had not yet begun to unite, and were 

 only closely approximated in the cephalic region. There are five 

 pairs of protovertebrte. At this stage there is no appearance of a 

 neurenteric canal ; the notochord passes directly behind into the 

 substance of the axial plate, which is still of considerable extent. 

 There is no mesoblast in the region in front of the head. The 

 head-folds of the splanchnopleure have become united in the 

 region of the head to form a short fore-gut. In the diverging 

 splanchnopleure folds there is yet no rudiment of the heart. 



