THE DKVELOPMENT OF THE MOLE. 125 



exist below the neural canal, closely approximated to the slight 

 rod-like notochord (figs. 24 and 26), while in the region of the 

 protovertebrse the mesoblast is more compact, and does not 

 extend so far beneath the medullary canal (fig. 28). 



In Stage j the anterior protovertebrse exhibit still further 

 changes : — (1) The vertebral portion of the somite has increased 

 very considerably in depth ; (2) the cavity has almost entirely 

 disappeared, remaining only as a mere slit (fig. 47) within ; (3) 

 the muscle-plate, which is now formed of two rows of columnar 

 cells. The second row lies inside the first, close beside and 

 parallel to it. It is formed from the cells of the vertebral por- 

 tion of the somite, which have hitherto occupied this position. 

 The two rows are continuous with each other at their dorsal 

 and ventral ends, and the cavity before spoken of lies between 

 them, reduced to a narrow slit. 



Posterior to the three anterior protovertebrse the muscle- 

 plate consists of only a single layer of columnar cells, as was 

 the case in the earlier stage (h) . 



The muscle-plates are therefore first formed anteriorly. 

 When examining this stage my attention was drawn to the 

 histological characters of the cells of the outer layer of the 

 muscle-plate in the anterior protovertebrse. 



These cells were observed with an ordinary Zeiss D lens 

 to be continued outwards into more or less fine processeSj and 

 upon examining sections with a high power (Powell and Lea- 

 land's ^th oil immersion and Reichert's y'g-th oil immersion) 

 it was found that these fine processes were branched or simple 

 prolongations of the mesoderm cells, which on the one hand 

 joined with the ectoderm cells, and on the other formed a 

 fine network immediately below the ectoderm. 



These processes are voluntary muscular fibres, which are thus 

 early developed from the outer portion of the muscle-plate. 

 This structure is fairly satisfactorily represented in fig. 51. 



The fact being observed that these mesoderm cells actually 

 joined the ectoderm cells led me to make a renewed exami- 

 nation of my sections of earlier stages, and I found that from 

 the time the hypoblastic mesoblast was formed in Stage c 



