326 DEVELOPMENT OF ELASMOBRANCH FISHES. 



increase in size and send prolongations downwards and inwards 

 to meet below the notochord. 



These soon become indistinguishably fused with other cells 

 which appear in the area between the alimentary cavity and the 

 notochord, but probably serve alone to form the vertebral bodies, 

 while the cells adjoining them form the basis for the connective 

 tissue of the kidneys, &c. 



The vertebral bodies also send prolongations dorsalwards 

 between the sides of the spinal cord and the muscle-plates. 

 These grow round till they meet above the spinal and enclose 

 the dorsal nerve-roots. They soon however become fused with 

 the dorsal prolongations from the muscle-plates, at least so far 

 as my methods of investigation enable me to determine ; but it 

 appears to me probable that they in reality remain distinct, and 

 become converted into the neural arches, while the connective- 

 tissue cells from the muscle-plates form the adjoining subcutaneous 

 and inter-muscular connective tissue. 



All the cells of the vertebral rudiments become stellate and 

 form typical embryonic connective-tissue. The rudiments how- 

 ever still retain their primitive segmentation, corresponding with 

 that of the muscle-plates, and do not during this period acquire 

 their secondary segmentation. Their segmentation is however 

 less clear than it was at an earlier period, and in the dorsal 

 part of the vertebral rudiments is mainly indicated by the dorsal 

 nerve-roots, which always pass out in the interval between two 

 vertebral rudiments. Vide PI. 12, fig. 12 pr. 



Intermediate Cell-mass. At about the period when the 

 muscle-plates become completely free, a fusion takes place be- 

 tween the somatopleure and splanchnopleure immediately above 

 the dorsal extremity of the true body-cavity (PL n, fig. 6). 

 The cells in the immediate neighbourhood of this fusion form 

 a special mass, which we may call the intermediate cell-mass 

 a name originally used by Waldeyer for the homologous cells 

 in the Chick. Out of it are developed the urino-genital organs 

 and the adjoining tissues. At first it forms little more than a 

 columnar epithelium, but by the close of the period is divided 

 .into (l) An epithelium on the free surface ; from this are derived 

 the glandular parts of the kidneys and functional parts of the 

 genital glands ; and (2) a subjacent stroma which forms the 



