34 ZOOLOGY sect. 



shape; these cells, which form the earliest rudiiiient of the nervous 

 system, become arranged in the form of a plate— the medullary 

 'plate— on the dorsal surface. On the surface of this plate appears 

 a groove — the medullary yroove — bounded by right and left medul- 

 lary folds, which pass into one another behind the blastopore. 



The medullary folds grow upwards and inwards over the medul- 

 lary groove, and unite together (Z>), the union beginning behind and 

 progressing forwards in such a way as to form a canal, the ncuro- 

 cmle, in the hinder portion of which is the opening of the blastopore. 

 In this process of closing-in of the medullary groove the fold which 

 passes round behind the blastopore takes an important part, grow- 

 ing forwards over the posterior portion of the canal. The blasto- 

 pore, thus enclosed in the medullary canal, persists for a time as a 

 small opening — the neurenteric canal — by which the neuroccele and 

 enteric cavity are placed in communication. At the anterior end 

 of the medullary canal, owing to its incomplete closure in this 

 region, there remains for a time an opening — the ncuropore (Fig. 

 734, neur.) — leading to the exterior. 



A notochoo'd (Fig. 733, 0, D, 734 and 735, nolo.) is formed from 

 certain of the cells of the wall of the archenteron along the middle 

 line of the dorsal side. These are arranged to form an 

 elongated cord of cells which becomes completely constricted off 

 from the endoderm of the wall of the archenteron, and comes to lie 

 between the latter and the medullary groove. Laterally certain 

 cells of the endoderm, in close relation to those forming the rudi- 

 ment of the notochord, divide to give rise to a pair of longitudinal 

 strands of cells— the rudiments of the mesoderm (Fig. 734, mcs.). 

 During this process of mesoderm-formation, there are no diverticula 

 developed from the archenteron. 



The embryo (Fig. 734, B) now becomes pear-shaped, the narrow 

 part being the rudiment of the future tail. As this narrow portion 

 elongates, the part of the enteric cavity which it contains soon 

 disappears, coming to be represented only by a strand of endoderm 

 cells, which gives rise in the middle to the extension backwards 

 of the notochord, laterally to the mesoderm of the tail, and ventrally 

 to a cord of endoderm cells continuous with the wall of the enteric 

 cavity in front. 



The caudal region increases in length rapidly, and the anterior 

 or trunk region, at first round, becomes oval. At its anterior end 

 there appear three processes of the ectoderm, the rudiments of the 

 adhesive papillcv (Fig. 735, adh.), organs by which the larva subse- 

 quently becomes fixed. The ectoderm cells at an early stage 

 secrete the rudiments of the cellulose test; in the caudal region 

 this forms longitudinal dorsal and ventral flaps having the function 

 of unpaired fins. 



The medullary canal becomes enlarged at its anterior end. A 

 vesicular outgrowth from this enlarged anterior portion forms the 



