BY WILLIAM A. HASWELL, M.A., D.SC. 591 



In the next stage observed, the head has become distmctly folded 

 off, there is a rudiment of the tail-fold, and there are two pairs of 

 protovertebrse ; the anterior or cephalic fold of the amnion is 

 rising up around the head, and the caudal fold is distinguishable, 

 though not prominent. The neural canal is closed throughout 

 except a very small portion at the posterior end ; the cerebral 

 vesicle presents no trace of subdivision, and the medullary canal 

 is not prolonged backwards as it is at a subsequent stage. 



The next stage (plate X, fig. 5) was from an egg which had been 

 incubated for sixty-five hours. The whole blastoderm was seven 

 and a half centimetres in diameter. The medullary canal was 

 closed throughout, the primary vesicles of the brain distinguish- 

 able, with rudiments of the optic vesicles and a commencing 

 division of the hind-brain into two. 



In an embryo of ninety-four houi's, which corresponds very 

 closely with a thirty-six hours' chick, there are sixteen pairs of 

 protovertebrae and the primaiy vesicles of the bi'ain are distinguish- 

 able ; the heart has the form of an S-shaped tube, and " blood- 

 islands" have begun to make their appearance on the future vascular 

 area. The medullary canal is nearly completely closed ; behind 

 (plate X. fig. 6) it is continued backwards in the form of a pear- 

 shaped structui'e — the remains of the primitive streak, such as is 

 often to be seen in a corresponding stage in the chick. At this 

 point, as is seen from sections, the notochord terminates by 

 becoming merged in what remains of the primitive streak ; the 

 hinder end of the medullary canal sends a short prolongation 

 downwards into the mass of cells constitutins: the remains of the 

 primitive streak, but this downward prolongation is short and ends 

 blindly below. It is the only representative of the neurenteric 

 canal found at a later stage. Behind it the three layers are all 

 united in the middle line for a short distance. 



There is still only a very thin layer of mesoblast in the region 



in front of the head. The fore-brain presents the merest rudiments 

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