26 AETHUE DENDY. 



membeied, was continuous in the preceding stages across the 

 middle line beneath the medullary plate. This mode of origin 

 is very clearly shown in the section represented in fig. 33, and 

 also in the sections represented in figs. 31 and 32. As we 

 trace it towards the anterior end the forward growth of primi- 

 tive-streak mesoblast gradually dies out, and so also does the 

 notochord, which gradually narrows and finally disappears 

 somewhere between the point where the head end of the embryo 

 becomes free from the overlying blastoderm, and the point to 

 which the anterior closure of the alimentary canal has ex- 

 tended (videfigs. 25— 32). 



Beneath the notochord the hypoblastic lining of the alimen- 

 tary canal has not yet assumed its characteristic short columnar 

 form, even anteriorly. 



The medullary groove at this stage has extended backwards 

 as far as the primitive streak, and has become continuous with 

 the neurenteric canal or blastoporic passage, which, however, 

 has not yet acquired its ventral opening (see figs. 31, 32). 

 The columnar cells lining the neurenteric canal appear to be 

 ciliated, but this appearance may be due to post-mortem changes. 

 The blastopore is no longer recognisable as a distinct structure, 

 its lips having become confluent with those of the medullary 

 groove in front of it. Posteriorly, however, in embryo 64 a 

 longitudinal furrow, not lined by columnar cells, is continued 

 backwards from the end of the medullary groove for some 

 distance over the surface of the primitive streak (fig. 34). This 

 furrow, like the similar one noticed in front of the blastopore 

 at the preceding stage (fig. 12), probably represents a portion 

 of the primitive groove, which would thus seem to extend both 

 in front of and behind the blastopore. 



Tracing the medullary groove forwards (figs. 21 — 22) we 

 find that although it has become very deep, and its lips are 

 approaching one another, yet it is still open for either the 

 whole or the greater part of its length. In both embryos it is 

 greatly dilated in front (figs. 17, 20, 21) to form the rudiment 

 of the brain, and in No. 64 this rudiment is already differen- 

 tiated into fore-, mid-, and hind brain, the latter being repre- 



