FORMATION OF THE MEMBRANES 801 



nasal cavities and lower part of the rectum ; the brain and spinal 

 cord; the organs of special sense, and possibly some parts of the 

 genito-urinary apparatus. The hypoblast is developed into the epithe- 

 lium lining the mucous membrane and glands of the digestive tract ; the 

 epithelium of the respiratory organs; the epithelium of the bladder and 

 urethra. There is a thickening of both these layers at the line of de- 

 velopment of the cerebro-spinal system, with a furrow that is finally 

 enclosed by an elevation of the ridges and their union posteriorly, form- 

 ing the canal for the spinal cord. 



As the spinal canal is developed, a new layer of cells is formed be- 

 tween the epiblast and the hypoblast, which is called the mesoblast. 

 The mesoblast itself afterward splits into two layers. All the parts not 

 enumerated as developed from the epiblast or hypoblast are developed 

 from the two layers of the mesoblast. The outer layer of the mesoblast, 



Fig. 223. Transverse section of the embryonic area of a dog's ovum of about fifteen days (Bonnet). 



This figure is introduced especially to show the mesoblast. The section is through the 

 head-fold (Chp.) ; M, mesoblast. 



or the epiblastic mesoblast, unites with the epiblast, and the two mem- 

 branes together form what is sometimes called the somatopleure ; from 

 which are developed the bones, muscles and external parts generally. 

 The inner layer of the mesoblast, the hypoblastic mesoblast, unites with 

 the hypoblast to form what is called the splanchnopleure ; from which 

 are developed the circulatory and glandular systems and the internal 

 parts generally. The cells lining the vessels, including the lymphatics, 

 which exist in a single layer, are called endothelial cells. This name 

 also is applied to the cells lining the serous membranes. 



FORMATION OF THE MEMBRANES 



In the mammalia a portion of the blastoderm is developed into mem- 

 branes by which a communication and union are established between 

 the ovum and the mucous membrane of the uterus. From the ovum two 

 membranes are developed ; one non-vascular, the amnion, and another, 



