34 GENERAL CONCEPTIONS. 



oping embryo. First, there is a special relation established between the embryo 

 and the uterus by means of a complicated adjustment of embryonic and uterine 

 tissues, which supplies nutrition to the embryo from the blood of the mother. 

 Second, there are the mammary glands, which also serve the same function. By 

 these two devices the embryo is even more completely freed from the necessity 

 of seeking its food and protecting itself than is the case with those forms, such as 

 the birds or elasmobranchs, in which the supply of food material is very large. 



Qerm=layers. 



The germ-layers are the first groups of cells to arise as the result of the seg- 

 mentation of the ovum. They are three in number, and each forms a distinct 

 sheet or lamina. As stated on page 26, these three primitive layers are termed 

 "ectoderm," "mesoderm," and "entoderm." The ectoderm is the most exter- 

 nal of the three, and upon the outside of the body parts of the ectoderm remain 

 permanently to constitute the outside skin or epidermis. From its very position 

 it necessarily is the part of the body to come into relation with the external 

 world, and accordingly we find that its two great duties are to produce the pro- 

 tective covering of the body and the apparatus for receiving and utilizing sensa- 

 tions; in other words, the chief sensory organs and the nervous system. The 

 entoderm, on the contrary, forms the internal cavity of the digestive canal and 

 its appendages. It therefore is concerned chiefly with the production of the or- 

 gans of digestion, and appears in the adult as the epithelium of the digestive and 

 respiratory organs and of the glands appended to the digestive tract. The 

 mesoderm, lying as it does between the other two layers, is shut off by them 

 from direct relation with the external world or with food-matter, and is accord- 

 ingly restricted to a series of internal functions, of which four are especially 

 important: the function of movement, of supporting the body, especially the 

 parts produced from the ectoderm and entoderm, of circulation, either of blood 

 or of lymph through definite channels, and, finally, of excretion. It is from the 

 middle germ-layer, therefore, that the muscular tissues arise, that the connective 

 and skeletal tissues arise, that the blood, blood-vessels, and lymphatics arise, 

 and that the excretory organs arise. 



The inner and outer germ-layers are primarily simple epithelial structures, 

 consisting each of a single layer of cells. This primitive characteristic is never 

 wholly obliterated and really controls all of the modifications which these two 

 layers undergo. The mesoderm, on the other hand, is primarily not epithelial, 

 but mesenchymal. Mesenchyma consists of widely separated cells which form a 

 continuous network of protoplasm, the meshes of which are originally filled by a 

 homogeneous intercellular substance or matrix. The student will have frequent 

 occasion in his practical work to study it in its embryonic stages. 



