18 GENERAL CONCEPTIONS. 



Germ-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 10, these three primitive layers are termed "ecto- 

 derm," "mesoderm," and "entoderm. " The ectoderm is the most external 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 protective covering of the body and the 

 apparatus for receiving and utilizing sensations; in other words, the chief sensory or- 

 gans 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 organs 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 accordingly restricted to a series of internal functions, of which four 

 are especially important: (i) The function of circulation both of blood and lymph 

 through definite channels; (2) of excretion; (3) of movement; (4) of supporting the 

 body, especially the parts produced from the ectoderm and entoderm. It is from 

 the middle germ-layer, therefore, that the connective and skeletal tissues arise, that 

 the muscular tissues arise, that the excretory organs arise, and that the blood, 

 blood-vessels, and lymphatics arise. 



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

 sisting 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 mesen- 

 chymal. 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. 



The Caelom. The ccelom is the primitive body-cavity of the embryo. It arises 

 as a space in the mesoderm. Soon after this space has appeared we find that the 

 cells of the mesoderm, which bound it, assume an epithelial character; consequently 

 the mesoderm, after the coelom has appeared, consists of mesenchyma and of an 

 epithelial layer bounding the coelom. This epithelial layer is called the mesothelium. 

 The mesoderm, therefore, differs fundamentally from the ectoderm and entoderm 

 by this peculiarity, that it comprises both an epithelial and a non-epithelial portion. 

 Both portions play very important roles in the production of the various tissues and 

 organs of the body. There is another respect also in which the mesoderm differs 

 from the other germ-layers, for we find that it increases in volume and in com- 

 plexity as we asced from the lower to the higher types of animals, or as we. pass 



