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 (ccelomic 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 tw r o 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 

 for a while. 

 When the pos- 



mastouerm wun emoryo in wnicii enuocierm ana mesouerm are , , i 



distinctly formed, and in which the alimentary slit has appeared. tenor part C1OS6S, 



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 ; ch. notochord ; 



ep. ectoderm ; m. mesoderm ; n. nuclei of yolk ; nc. neuroccele; 



sg. segmentation-cavity ; x, point where ectoderm and endoderm 



become continuous at the posterior end of the embryo. 



Balfour.) 



FIG. 852. Diagrammatic longitudinal sections of an Elasmo- 

 branch embryo. A, 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 



so in 



a way that it 

 encloses the blas- 

 topore, and there 

 is thus formed, 

 (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. 



