THE FCETAL APPENDAGES. 7- 



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

 sal laminse forms the ventral lamince ; and these bend down- 

 wards and inwards, at a short distance on either side of the 

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

 tube. The ventral laminaj 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, extend- 

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

 the fii-st-formed and the most constant of the temporary, 

 or foetal, appendages of the young vertebrate, the mnbilical 

 vesicle. 



While these changes are occun-ing, 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 meanwhile, by histological differen- 

 tiation of the axial indifferent tissue, immediately under 

 the floor of the primitive groove), into two lamellae. One of 

 these, the visceral lamella, remains closely adherent to the 

 hypoblast, forming with it the splanchnopleure, and eventu- 

 ally becomes the proper wall of the enteric canal; while 

 the other, the parietal lamella, follows the epiblast, forming 

 with it the somatnpleure, 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 umbilicus. 



The walls of the cavity formed by the splitting of the 

 ventral laminae acqiiire an epithelial lining, and become the 

 gre&t pleuroperitoneal serous membranes. 



The Foetal Appendages of the Vertebrata. — At its outer 

 margin, that part of the somatopleure which is to be 

 converted into the thoracic and abdominal wall of the 

 embryo, grows up anteriorly, posteriorly, and laterally, 

 over the body of the embryo. The free margins of 

 this fold gradually approach one another, and, ulti- 

 mately uniting, the inner layer of the fold becomes 

 converted into a sac filled with a clear fluid, the .4171- 

 nion ; while the outer layer either disappears, or coa- 



