XIII 



PHYLUM CHORDATA 



169 



layer cells and assumes the form of a pair of independent plates 

 one on either side of the middle line of the body. 



As the blastoderm extends over the yolk the edge forms a ridge 

 continuous with the embryonic rim. The latter assumes the form 

 of two prominent caudal swellings (Fig. 794, cd.). The medullary 

 groove meanwhile deepens, and its edges grow over, as in Amphi- 

 oxus and the Urochorda, so as to form a canal (Fig. 793, C: 

 Fig. 795). The union takes place first in the middle, the anterior 



Fit;. 703. Diagrammatic longitudinal sections of an Elasmobranch embryo. -/, section of 

 the young blastoderm with segmentation-cavity enclosed in the lower layer cells. B, older 

 blastoderm with embryo in which endoderm and mesoderm are distinctly formed, and in 

 which the alimentary slit has appeared. The segmentation-cavity is still represented as 

 being present, though by this stage it has in reality disappeared. C\ older blastoderm with 

 embryo in which the neural canal has become formed and is continuous posteriorly with the 

 alimentary canal. Ectoderm without shading ; mesoderm and also notochord black with clear 

 outlines to the cells ; endoderm and lower layer cells with simple shading, al. alimentary 

 cavity ; <"/< . notochord ; ep, ectoderm ; m. mesoderm ; n. nuclei of yolk ; nc. neurocoele ; .SY/. 

 segmentation-cavity ; x. point where ectoderm and endoderm become continuous at the" 

 posterior end of the embryo. (From Balfour.) 



and posterior parts (Fig. 795, neur.) remaining open for a while. 

 When the posterior part closes, it does so in such a way that it- 

 encloses the blastopore, and there is thus formed, as in the 

 Ascidian, a temporary passage of communication between the 

 medullary canal and the archenteron the neur enteric passage. 



The ectoderm gives rise, as in Vertebrates in general, not only 

 to the epidermis and the central nervous system, but also to the 

 peripheral nervous system, the lining membrane of the olfactory 

 sacs, the lens of the eye, and the lining membrane of the auditory 

 labyrinth, of the mouth and gill clefts, and of the cloaca. 



