24:0 PRINCIPLES OF STOCK-BKEEDING. 



layer ; and it is from these cells that the greatest part 

 of the embrjon is formed." * 



As there is a tendency to a subdivision of the three 

 layers mentioned, some modern physiologists include 

 in the external layer the upper surface of the inter- 

 mediate layer, while the lower surface of the inter- 

 mediate layer is included in the inner layer of the 

 blastodermic membrane. 



This does not, however, involve any difference of 

 opinion as to the parts of the germinal membrane that 

 are developed into the different organs of the body. 

 Dr. Marshall, who adopts the latter classification, says : 

 " From the upper external or serous layer, also named 

 the sensorial layer, are developed, along its axial por- 

 tion, the cerebro-spinal nervous axis and the organs of 

 the senses, and, from its lateral portions, the cuticle 

 or outer skin, with its epidermic appendages, the 

 feathers, bill, and claws, and, in the mammalia, the 

 nails and hairs ; lastly, the sebaceous and sudoriferous 

 cutaneous glands, and the Meibomian, ceruminous, 

 and mammary glands. 



" From the middle layer, also called the motorio- 

 sexual layer, are developed, by complicated metamor- 

 phoses of its substance, the bones, the muscular sys- 

 tem, the peripheral spinal nerves, the sympathetic 

 nerves, the heart, blood-vessels, and lymphatic system, 

 the so-called ductless glands, and the reproductive 

 organs ; also, next to the external layer, the true skin, 

 and, next to the internal layer, the muscular and sub- 

 mucous coats of the alimentary canal. 



"Lastly, from the internal layer, also called the 



1 Flint's "Physiology," 1875, vol. v., p. 360. 



