DEVELOPMENT. 187 



the head and tail folds. The longitudinal folds define the outlines 

 of the body. 



How does the medullary groove develop ? 



The ridges in the epiblast on either side continue to thicken, and 

 finally coalesce at the back, leaving a lining of cells within a tube. 

 These cells develop the cerebro-spinal axis and follow the curves 

 of the blastoderm longitudinally. The head fold gradually becomes 

 constricted, and becomes the neck. Meanwhile the mesoblast has 

 developed a number of processes, which become a primitive spinal 

 column. It is to be noticed that the enclosure of the spinal canal 

 in this way is by a folding process, and all through the process of 

 development this peculiarity is very marked. 



What is the mode of the formation of the pleuro-peritoneal cavity ? 

 The mesoblast divides near the median line, and one part of the 

 split mesoblast adheres to the epiblast, while the remainder joins 

 the hypoblast. The former is known as the somatopleure, and the 

 latter as the splanchnopleure. and the included space becomes the 

 pleuro-peritoneal cavity, which subsequently divides to form pleura, 

 pericardium, and peritoneum. 



What structures are developed from the epiblast? 



The epidermis and appendages of the skin, the great nervous 

 centres, the nerves, and the principal portions of organs of special 

 sense eye. ear, nose. 



What does the hypoblast develop ? 



The epithelial elements, including the glands and the mucous 

 membranes lining the alimentary and pulmonary tracts. 



What does the mesoblast develop? 



The bones, muscles, fasciae, and connective tissues of the body. 

 It also develops the vascular system. It must be remembered that 

 very early in the embryonal life this tissue divides to join with the 

 epiblast and hypoblast, and we can therefore understand how it 

 may develop the muscular structures of the intestinal canal. 



What is the destiny of the splanchnopleure ? 



The splanchnopleure (5', Fig. 59) folds in over the remainder of 

 the ovum, and so walls off, so to speak, a portion of the vitellus which 

 communicates freely with the rest of the yelk-sac. Later on, after 



