THE MICROSCOPE IN ANIMAL HISTOLOGY. 203 



this, cut off from the rest, becomes thickened, and takes 

 on the structure of the brain, or encephalon, 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 

 dorsal laminae forms the ventral lamince ; and these bend 

 downward 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 sur- 

 faces, and thus, in most cases, tend to constrict off the 

 central from the peripheral portions of the blastoderm. 

 The latter, extending over the yelk, incloses it in a kind 

 of bag. This bag is the first formed and the most con- 

 stant of the temporary, or foetal appendages of the young 

 vertebrate, the umbilical vesicle. 



While 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 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 

 eventually becomes the proper wall of the enteric canal ; 

 while the other, the parietal lamella, 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 somato- 

 pleures eventually unite, is the 'umbilicus. 



The walls of the cavity formed by the splitting of the 

 ventral laminae acquire an epithelial lining, and become 

 the great pleuroperitoneal serous membranes (Huxley's 

 Anatomy of Vertebrated Animals). 



In addition to the umbilical vesicle, above described as 



