802 GENERATION. 



the ovum presents an analogy to the albuminous coverings which the eggs of 

 oviparous animals receive in the oviducts; with the difference that this 

 albuminous matter is almost the sole source of nourishment in the latter, 

 and exists in large quantity, while in viviparous animals, the quantity is 

 small, is generally consumed as the ovum passes into the uterus, and in the 

 uterus, the ovum forms attachments to and draws its nourishment from the 

 vascular system of the mother. 



Soon after the formation of the single, blastodermic membrane, at a cer- 

 tain point on its surface there appears a rounded elevation or heap of smaller 

 cells, forming a distinct spot, called the embryonic spot. As development 

 advances, this spot becomes elongated and oval. It is then surrounded by a 

 clear, oval area, called the area pellucida, and the area pellucida is itself sur- 

 rounded by a zone of cells, more granular and darker than the rest of the 

 blastoderrn, called the area opaca. The line thus formed and surrounded by 

 the area pellucida is called the primitive trace. This primitive trace, or 

 primitive groove, however, is a temporary structure. After the groove is 

 formed, there appears in front of but not continuous 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 developed into the neural canal. 



Blastodermic Layers. The blastodermic cells, resulting originally from 

 the segmentation of the vitellus, are first split apparently into two layers, the 

 external, or epiblast, and the internal, or hypoblast. The epiblast is devel- 

 oped 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-urinary apparatus. The 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 eleva- 

 tion 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 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 meso- 

 blast. 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, 



