108 VERTEBRAL BODIES. 



typical branclied connective-tissue cells before tlie close of 

 the period (PI. x. fig. 7 and 8). 



Between stages I and K the bodies of the vertebrse rapidly 

 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 con- 

 nective-tissue cells from the muscle-plates form the adjoining 

 subcutaneous and inter-muscular connective tissue. 



All the cells of vertebral rudiments become stellate and 

 form typical embryonic connective tissue. The rudiments 

 however 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. xi. 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 X. 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 



