178 



ZOOLOGY 



SECT. 



(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 (coelomic bays) from which their development takes its 

 origin may represent the cavities of the ccelomic 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 two prominent 

 caudal swellings 

 (Fig. 853, cd.). 

 The medullary 

 groove mean- 

 while deepens, 

 and its edges 

 grow over so as 

 to form a canal 

 (Fig. 852, C ; Fig. 

 854). The union 

 takes place first 

 in the middle, 

 the anterior and 

 posterior parts 

 (Fig. 854, neur.) 

 remaining open 



FIG 852. -Diagrammatic longitudinal sections of an Elasmo- f nr Q w 1, ; 1 

 branch embryo. A, section of the young blastoderm with L e - 



segmentation-cavity enclosed in the lower layer cells ; B, older When the T)OS- 



blastoclerm with embryo in which endoderm and mesoderm are , 



distinctly formed, and in which the alimentary slit has appeared tenor part Closes, 



The segmentation-cavity is still represented as being present,' 'f A^v t 



though by this stage it has in reality disappeared. C, older lt} does SO m sucn 



blastoderm with embryo in which the neural canal has become a W flV tVmt if 



formed and is continuous posteriorly with the alimentary canal. ' * 



Ectoderm without shading ; mesoderm and also notochord black encloses the blas- 

 with clear outlines to the cells ; endoderm and lower layer cells 



with simple shading, al. alimentary cavity ; ch. notochord topore, and there 



ep. ectoderm ; TO. mesoderm; n. nuclei of yolk; nc. neurocoele;' ; fV, 1iq f nrmpr j 



sff. segmentation-cavity ; x, point where ectoderm and endoderm x 3a > 



BaSour ) ntinuous at the P sterior end of the embryo. (From as in AmphioXUS 



and Ascidians, a 



temporary passage of communication between 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. 



