DEVELOPMENT. 21 



dorsal side of the already bilaterally symmetrical embryo 

 extending from the blastopore forwards. This groove, into the 

 hind end of which the blastopore opens, is the first rudiment of 

 the central nervous system. It is known as the medullary 

 groove. Its edges project and form the medullary folds which 

 grow round and close the narrow blastopore, and gradually unite 

 with one another from this point forward in such a manner as to 

 convert the groove into a canal, the walls of which separate from 

 the external ectoderm and give rise to the central nervous system. 

 This canal is the medullary canal ; behind it is shut off from the 

 exterior, but communicates with the cavity of the gastrula by 

 way of the blastopore which is now called the neurenteric canal ; 

 while in front it remains open for some time, but eventually 

 closes. Before these changes are completed, the medio-dorsal 

 endoderm cells of that part of the gastric wall which immediately 

 underlies the posterior part of the neural canal (Fig. 15, c and d, 

 Ch) become different from the remaining endoderm cells and 

 constitute the first rudiment of the notochord. Meanwhile the 

 latero-dorsal endoderm, on each side of the notochord. has 

 saparated off the mesoderm as a solid * plate of cells 

 (Fig. 16, ms). These, in the trunk, later become converted into 

 a mesenchyme, occupying the space 

 which now makes its appearance be- 

 tween the ectoderm and endoderm, 

 and give rise to the blood- corpuscles, 

 musculature, genital, and excretory 

 organs of the body, while the caudal 

 part becomes the musculature of the 

 tail. The distinction between the 

 caudal and trunk region of the em- 

 bryo is now very apparent. The 

 notochord is confined to the tail. 

 The caudal part of the enteron be- FlG . i 6 ._ T raiverse section through 



nrkrnoa cr^lirl /T^i'rr 1^/7 #V7 ' \ fVirmn-Vi an embr y of Clavelina (after V. 



COmeS SOllQ (.Fig. L5a,fia ), tnOUgll Beneden andJulin, from Korschelt 



still remaining continuous with the 

 medullary tube round the hind end 



of the notochord. Eventually it 



disappears, apparently giving rise to blood corpuscles. 



* According to van Beneden and Julin there is at first an enteric pro- 

 longation in the front part of this. 



