258 COMPARATIVE ANATOMY 



blastopore. A thickening is formed on either side of the 

 anterior end of the neural folds, and each thickening after 

 a time is divided into two by a transverse furrow. The 

 part lying behind the furrow is then known as the gill-plate, 

 the part lying in front of it as the sense-plate. The sense- 

 plates eventually grow downwards and forwards, and meet 

 in front of the neural folds. The latter increase in height, 

 and begin to fold in towards the median line, forming the 

 walls of a deep neural groove, which extends posteriorly to 

 the blastopore. The embryo now begins to elongate; the 

 neural folds become longer and deeper, and turn inwards 

 towards the middle line, while posteriorly they overlap and 

 begin to enclose the blastopore. The elongation of the 

 embryo continues, the neural folds meet and fuse in the 

 middle line, their union being first effected about the middle 

 of their length, and extending forwards and backwards. Thus 

 they convert the neural groove into a tube, which becomes 

 the central canal of the spinal cord and the ventricles of 

 the brain. Before the folds have united in front, the anterior 

 end of the neural groove is enlarged, and even before they 

 have met and fused this bulb-like enlargement is divided 

 by slight constrictions into a fore-brain, a mid-brain, and 

 a hind-brain. Posteriorly the neural folds envelop and fuse 

 together over the blastopore, and as the latter opens into 

 the enteron a neurenteric canal is established, having the same 

 relations as in Amphioxus. It is a transitory structure, and 

 soon closes up. At this time the gut is completely closed, 

 neither mouth nor anus being present ; but the latter is soon 

 formed as a small depression situated a little way behind 

 the blastopore, and connected with it by a groove. The 

 depression grows deeper, and meets an outgrowth from the 

 floor of the hinder end of the enteron, their walls fuse and 

 are perforated to form the anus. At first the anus is nearly 

 on the dorsal side of the embryo, but soon after its formation 

 the region above grows out to form the commencement of 

 the tail, and the anus is then shifted to a ventral position. 

 Meanwhile important changes have been going on inside the 

 ovum. We have already seen that the internal layer of cells 

 forming the roof and walls of the enteron becomes split at 

 an early stage into a thin sheet of hypoblast, forming the 

 actual roof and side walls of the enteron, and a thicker sheet 



