202 THE MICROSCOPIST. 



tissue. This rudimentary embryonic tissue consists of 

 three layers of cells, or germinal plates. The upper is the 

 corneous layer, or epiblast, the middle one the intermediate 

 plate, or mesohlast, and the lower the intestinal glandular 

 layer, or hypoblast (Plate XXI, Fig. 160). From these 

 the various tissues and organs are developed. 



The outer plate produces the epithelium of the skin and 

 its appendages, with the cellular elements of the glands of 

 the skin, mammae, and lachrymal organs. By a peculiar 

 folding over the axis this plate also produces the elements 

 of the brain and spinal cord, and the internal parts of the 

 organs of special sense. The physiological significance of 

 this layer is, therefore, very great. 



The lower stratum of the blastoderm supplies the epi- 

 thelium of the digestive tract, and the cellular constituents 

 of its various glands, together with the liver, lungs, and 

 pancreas. 



The middle layer supplies the material for many struc- 

 tures. The whole group of connective substances, or 

 tissues of support; muscular tissue; blood and lymph, 

 with their containing vessels ; lymph-glands, including 

 the spleen, etc., all arise from this. The epithelial cells 

 of such tubes and cavities as originate in this layer are 

 regarded as different from those of true glands, and are 

 more permeable to fluids. They have been, termed false 

 epithelium, or endothelium. 



The following description, by Professor Huxley, will 

 enable the student to form an idea of the general process 

 of development. A linear depression, the primitive groove, 

 makes its appearance on the surface of the blastoderm, 

 and the substance of the mesoblast along each side of this 

 groove grows up, carrying with it the superjacent epiblast. 

 Thus are produced the two dorsal lamince, the free edges 

 of which arch over toward one another, and eventually 

 unite, so as to convert the primitive groove into the cere- 

 bro-spinal canal. The portion of the epiblast which lines 



