184 



ZOOLOGY 



S3CT. 



The notochoRl (Fig. 841, cJi.) is developed as a cord of cells 

 derived from the lower layer. 



Each lateral sheet of mesoderm, soon after its formation by the 

 coalescence of the peripheral and central rudiments, becomes divided 

 by the development of a horizontally directed cleft-like space in its 

 interior. The inner part of each sheet then separates from the 

 outer by the formation of a longitudinal fissure. The former, which 

 is known as the vertchral plate, becomes divided by a series of trans- 

 verse fissures into a number of squarish masses, the jr/'o?^ai'cri!cZ/rcr; or 



Fig. S-U.— Diagrammatic loiiKitiidinal sections of an Elasmobranch embryo. A, section of 

 the yoniig Vilastoderm with segmentation-cavity enclosed in the lower layer cell, ; B, older 

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

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

 lieing present, though by this stage it has in i-eality disapjjeared. C, older blastoderm with 

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

 alimentary canal. Ectodfrm without shading ; mtsoderni and also notochord black with clear 

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

 cavity ; rl,. notochord ; ep, ectoderm ; m. mesoderm ; n. nuclei of yolk ; n.f neurociele ; sa. 

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

 posterior end of the embryo. (From Balfour.) 



mesodermal somitcfi. The outer part forms a broad plate, the lateral 

 inlate. The lateral plate consists of two layers, a dorsal or somatic, 

 and a ventral or qjlanrhnir, and the cavity between them is the 

 beginning of the coelome. The protovertebra> send off cells 

 round the notochord to form the bodies of the vertebrae, the 

 remainder giving rise to the muscles of the voluntary system. An 

 isthmus of mesoderm cells {neplirotome), which still connects each 

 proto\'ertebra with the lateral plate and contains a prolongation 

 of the cavity, gives rise to the pronepluic duct and tubules. The 



