146 ESSENTIALS OF ZOOLOGY 



activities of the lips of the blastopore. The cells which enter 

 into the formation of the dorsal plate gradually enlarge, while 

 the ectodermal cells immediately outside them decrease in 

 size. The neural plate cells not only enlarge, but the plate 

 is bent downwards, together with the endoderm below, with 

 the result that a shallow groove is produced, the medullary 

 or neural groove. As this is happening, the neural cells 

 become sharply marked off from the lateral ectoderm cells in 

 the neighbourhood of the blastopore. The latter then com- 

 mence to grow over the neural plate and ultimately meet and 

 fuse, thus shutting off the neural plate from the exterior. 

 The process begins in the neighbourhood of the blastopore 

 and extends forwards, but, anteriorly, an opening is left 

 which allows the neural groove to communicate with the 

 exterior. This opening is the neuropore, and there the neural 

 layer and the ectoderm of the body maintain their original 

 continuity. The procedure begins on each side of the narrow 

 blastopore, and as a result of the rising and fusion of the 

 ectoderm the blastopore is shut out from the exterior. The 

 blastopore is thereby converted into the neurenteric canal. 



Underneath the covering of ectoderm the neural plate is 

 rotated upwards laterally, and the two ridges meet and fuse 

 dorsally, thus converting the neural groove into the ciliated 

 neural canal. The neural canal is therefore a tube open in 

 front at the neuropore and communicating posteriorly by the 

 neurenteric canal with the enteron. The neurenteric canal 

 lies behind the differentiating notochord and persists to a late 

 period of growth. The neural plate by this process is gradually 

 converted into the nerve cord. 



The mesoderm is derived entirely from the dorsal part of 

 the endoderm. The median flattened part of the roof becomes 

 grooved, and the lips meet and fuse to form the notochord. 

 As it is gradually separated in antero-posterior direction the 

 cavity of the groove disappears, and the notochordal cells, at 

 first few in number, form a solid longitudinal cord situated 

 between the enteron and the nerve cord. The differentiation 

 begins when about eight somites are formed in the region 

 which marks the beginning of the postgastrular growth. 



The advent of the notochord is accompanied by that of 



