142 WALTER HEAPE. 



An examination of consecutive sections reveals, in front of 

 the proto vertebrae, the axial and peripheral mesoblast in the 

 form of a continuous solid plate, with no cavity in the axial 

 portion ; while in the peripheral portion the cavity gradually 

 recedes outwards (fig. 14) until it no longer exists within the 

 limits of the embryonic area. 



Behind the protovertebrae the cavity in both axial and peri- 

 pheral mesoblast becomes at once and simultaneously oblite- 

 rated, and two thick solid lateral plates of mesoblast extend 

 backwards, and join the mesoblast of the primitive streak. 



As the medullary groove closes in, the protovertebrae become 

 more cubical and compact (compare sections of Stage h, show- 

 ing these points (figs. 31, 30, and 28) ), and the narrow slit re- 

 duced to a small central pore, which about this time becomes 

 very generally partially filled up by a core of cells derived from 

 the lower and inner portion of the protovertebra. 



The protovertebrae then (Stage h) commence, first in the 

 anterior region, and gradually assuming in subsequent stages the 

 same relation posteriorly, to divide into two portions, an outer 

 and dorsal arched portion composed of columnar cells, and a 

 lower and inner portion formed of irregularly rounded cells (fig. 

 29; compare also fig. 52 of Stage j), the former giving rise 

 mainly to the muscle-plates, the latter to the bodies of the 

 vertebrae and the connective tissue surrounding them. It will 

 be shown subsequently, however, that the inner portion also 

 participates in the formation of the muscle-plate. 



A very marked cavity exists between the two portions on 

 the outer side of the somite (fig. 29), and the vertebral portion 

 of the mesoblast is continued ventrally below the neural canal 

 towards the notochord. 



The cavity is derived from the small cavity present in the 

 earlier stage (Stage e) ; and it is worthy of notice it is not first 

 obliterated and then again formed, as has been stated by some 

 observers to be the case in the Chick, nor does it entirely dis- 

 appear, as has been supposed to be its fate in Mammalia 

 {vide No. 1, p. 553). 

 Anterior to the protovertebrae scattered mesoblast cells 



