802 



GENEKATION, 



with it, a new fold and a groove leading from it. This is the " head-fold," 

 and the groove is the true medullary groove, which is subsequently devel- 



oped into the neural canal. 

 Blastodermic Layers. 

 The blastodermic cells, re- 

 sulting originally from the 

 segmentation of the vitel- 

 lus, are first split appar- 

 ently into two layers, the 

 external, or epiblast, and 

 the internal, or hypoblast. 

 The epiblast is developed 

 into the epidermis and its 

 appendages, the glands of 

 the skin, the brain and 

 spinal cord, the organs of 

 special sense and possibly 

 some parts of the genito- 



. + TK 



a, primitive trace; b, area pellucida ; c, area opaca ; d, blaeto- Urinary apparatus. IJlC 



re118 * *' *' ^ Beginning to appear on the vitelline 



FIG. 294. Primitive trace of the embryon (Liegeois). 



memrane hypoblast is developed into 



the epithelium lining the 



mucous membrane and glands of the stomach and intestinal canal. There 

 is a thickening of both of these layers at the line of development of 

 the cerebro-spinal system, with a furrow that is finally enclosed by an 

 elevation of the ridges and their union posteriorly, forming the canal for 

 the spinal cord. 



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

 the epiblast and the hypoblast, which is called the mesoblast. The meso- 

 blast itself afterward splits into two layers. All the parts not enumerated 

 as developed from the epiblast or hypoblast are developed from the two lay- 

 ers of the mesoblast. The outer layer of the mesoblast, or the epiblastic 

 mesoblast, unites with the epiblast, and the two membranes together form 

 what is sometimes called the somatopleure. The inner layer of the meso- 

 blast, the hypoblastic mesoblast, unites with the hypoblast x to form what is 

 called the splanchnopleure. The cells lining the vessels, including the 

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

 name is also 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 mem- 

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

 tois, which is vascular. The two layers of decidua are formed from the 

 mucous membrane of the uterus. At a certain part of the uterus, a vascular 

 connection is established between the mucous membrane and the allantois, 



