184 



ZOOLOGy 



SECT, 



The notochord (Fig. 841, ch.) is developed as a cord of cells 

 derived from the lower layer. 



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

 coalescence of the peripheral and central rudiments, becomes divided 

 by the development of a horizontally directed cleft-like sj)ace 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 vertebral plate, becomes divided by a series of trans- 

 verse fissures into a number of sq\ia.rish. masses, the j^rotovertchrceov 



I. 841.— Diagrammatic longitudinal sections of an Elasmobranch embryo. A, section of 

 the yoiing blastoderm with segmentation-cavity enclosed in the lower layer cella ; ]i, 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 disappeai-ed. C, older blastoderm with 

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

 alimentary canal. Ectodeiin without shading ; mesoderm and also notoehord black with clear 

 outlines to the cells ; endoderm and lower-layer cells with simple shading. aL alitoentiiry 

 cavity ; eh. notochord ; ej>, ectoderm ; m. mesoderm ; n. nuclei of yolk ; nx neurocoele ; eg. 

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

 posterior end of the embryo. (From Balfour.) 



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

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

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

 beginning of the coelome. The protovertebrse 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 (nephrotome), which still connects each 

 protovertebra with the lateral plate and contains a prolongation 

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



