XIII 



PHYLUM CHORDATA 



183 



ccelom.ic bay (c,.b. 2 ) is distinguishable, and from this a plate of 

 mesoderm cells grows outwards. Eventually the peripheral and 

 central plates of mesoderm come into contact and coalesce to form 

 a continuous sheet on each side of the middle line. Though 

 the mesodermal rudiments, peripheral and central, contain no 

 cavities, the grooves (coslomic bays) from which their development 

 takes its origin, may represent the cavities of the coelomic sacs of 

 Amphioxus. 



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

 continuous with the embryonic rim. The latter assumes the form of 



FIG. 840. FriSturus, transverse section of blastoderm, showing the formation of the mesoderm. 

 ?)/> I. dorsal lip of blastopore ; <. 6.1 external ccelomic bay ; c. !>.% internal coelomic bay ; 

 ec. ectoderm ; en. endoderm ; m. /. medullary fold ; HI. pr. medullary groove ; ws.l external 

 rudiment of mesodirm; wi*. 2 internal rudiment of mesoderm; nc. notochord ; y. yolk; 

 yl'.n. yolk nuclei. (From O. Hertwig, after Rabl.) 



two prominent caudal swellings (Fig. 842, 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. 841, G\ 

 Fig. 843). The union takes place first in the middle, the anterior 

 and posterior parts (Fig. 843, 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 

 Amphioxus and Ascidians, a temporary passage of communica- 

 tion betsveen the medullary canal and the archenteron the 

 neurenteric 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 of the outer portions of the cloaca and 

 gill-clefts. 



