PISCES 193 



the same time is raised from the blastoderm as the head fold, 

 and the fold marks permanently the anterior end of the embryo. 

 Both layers are raised, and the growth of the fold extends 

 forwards and upwards. Thus a pouch of the endoderm is 

 produced as the anterior end of the enteron. The embryonic 

 region between the head fold and the dorsal lip becomes 

 thereafter more pronounced by the flattening and grooving 

 of the ectoderm in the formation of the brain, and by a corre- 

 sponding flattening of the endoderm below in the formation 

 of the anterior end of the notochord. Sections show also 

 that the coelomic mesoderm is being budded off from the edge 

 of the flattened notochord. 



Growth is accelerated laterally in the region of the embryo, 

 forming and raising the ridges defining the neural groove, and 

 the growth extends to the posterior margin, where the two 

 ridges are continued into two prominences defining between 

 them a neurenteric groove. 



The medullary or neural groove is gradually raised and 

 fused, being converted into the neural canal. The canal 

 remains open for some time as a neuropore in front. 



At the posterior end of the embryo the groove remains 

 open until the time comes for the conversion of the neur- 

 enteric groove into the neurenteric canal. This is done by the 

 rotation ventrally of the prominences and their fusion all 

 round the posterior end of the embryo. The fusion forms a 

 tail fold containing a posterior portion of the enteron com- 

 municating by the neurenteric canal with the neural canal. 

 The line of fusion is the median line of the embryo, and the 

 fusion is continued below over the yolk, where the line of fusion 

 of the lips of the blastopore forms a primitive streak. 



There is a great deal of this which is similar to Amphioxus. 

 It is to be observed, however, that the fusion which produces 

 the neurenteric canal in Amphioxus completely obliterates 

 the blastopore, but in the Elasmobranchs it is only the upper 

 or dorsal part of the blastopore which is so converted. The 

 rest of the blastopore forms a line of fusion of ectoderm and 

 endoderm extending beyond the embryo. In both cases we 

 have an embryo the enteron of which is extended into the 

 neural canal and opens anteriorly by a neuropore. It is 



