xm PHYLUM CHORDATA 29 



the rudiment of the posterior portion of the notochord ; the lateral 

 parts go to form the caudal part of the mesoderm. 



The medullary folds grow upwards and inwards over the medul- 



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

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

 ccele, 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, 

 growing forwards over the posterior part 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 neuropore (Fig. 

 689, new.) leading to the exterior. 



The embryo (Fig. 689, B\ now becomes pear-shaped, the narrow 

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

 elongates the part of the enteric cavity which it contains soon 

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

 cells. In the anterior wide part of the embryo the mesoderm 

 (mes.) arises by the formation of paired outgrowths, which arise 

 from the dorsal wall of the archenteron. A row of endoderm cells 

 between the two sets of outgrowths represent the rudiments of 

 the trunk part of the notochord ; they become arranged to form 

 a cylindrical cord. 



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 papillae (Fig. 690, 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 

 sense-vesicle (sens. ves.). The posterior narrow part forms the caudal 

 portion of the central nervous system (spinal cord). Masses of 

 pigment in relation to the sense-vesicle early form the rudiment 

 of the two larval sense-organs, otocyst and eye. The part behind 

 this presents a thickened wall with a narrow lumen. This is 

 known as the ganglion of the trunk. The rudiment of the hypo- 

 physis early appears as a ciliated diverticulum (cil. gr.) of the 

 anterior end of the archenteron. 



The embryonic alimentary canal consists of two regions, a wide 

 region situated altogether in front of the notochord, and a nar- 

 rower portion situated behind in the region of the notochord. The 

 wider anterior part gives rise to the pharynx ; the posterior part 



