12 THE ANATOMY OF YERTEBEATED ANIMALS. 



fclie substance of the mesoblast along eacli side of this groove 

 grows up, carrying with it the superjacent epiblast. Thus are 

 produced the two dorsal lammce^ the free edges of which arch 

 over toward one another, and eventually unite, so as to con- 

 vert the primitive groove into the cerebro-spinal canal. The 

 portion of the epiblast which linps this, cut off from the rest, 

 becomes thickened, and takes on the structure of the brain, or 

 Eticephalon^ in the region of the head; and of the spinal 

 cord, or Myelon, in the region of the spine. The rest of the 

 epiblast is converted into the epidermis. 



The part of the blastoderm which lies external to the dor- 

 sal laminjB forms the ventral knnijice / and these bend down- 

 ward and inward, at a short distance on either side of the 

 dorsal tube, to become the walls of a ventral, or visceral, 

 tube. The ventral laminae carry the epiblast on their outer 

 surfaces, and the hypoblast on their inner surfaces, and thus, 

 in most cases, tend to constrict off the central from the 

 peripheral portions of the blastoderm. The latter, extending 

 over the yelk, encloses it in a kind of bag. This bag is the 

 first-formed and the most constant of the temporary, or foetal, 

 appendages of the young vertebrate, the umhilical vesicle. 



YV^hile these changes are occurring, the mesoblast splits, 

 throughout the regions of the thorax and abdomen, from its 

 ventral margin, nearly up to the notochord (which has been 

 developed, in the mean while, by histological differentiation of 

 the axial indifferent tissue, immediately under the floor of the 

 primitive groove), into tw^o lamelloi. One of these, the visceral 

 lamella^ remains closely adherent to the hypoblast, forming 

 with it the splanclinopleure^ and eventually becomes the proper 

 wall of the enteric canal ; while the other, the parietal latnella^ 

 follows the epiblast, forming with it the somatopleure^ which 

 is converted into the parietes of the thorax and abdomen. 

 The point of the middle line of the abdomen at which the 

 somatopleures eventually unite, is the umhilicus. 



The walls of the cavity formed by the splitting of the 

 ventral laminae acquire an epithelial lining, and become the 

 great pleuroperitoneal serous membranes. 



The Fodal Appendages of the Vertehrata. — At its outer 

 margin, that part of the somatopleure which is to be con- 

 verted into the thoracic and abdominal wall of the embryo, 

 grows up anteriorly, posteriony, and laterally, over the body 

 of the embryo. The free margins of this fold gradually ap- 

 proach one another, and, ultimately uniting, the inner layer 

 of the fold becomes converted into a sac filled with a clear 



