954 SPECIAL PHYSIOLOGY. 



longitudinal plates, named the laminae dor sales, which enlarge and 

 elongate, as the area itself becomes larger and pyriform in outline. 

 Beneath this groove, immediately below its floor, appears a delicate, 

 semi-opaque thread, at first cellular, but soon becoming cartilaginous, 

 named the chorda dorsalis or notochord, which part, so characteristic 

 of the Vertebrate type, is recognizable in the chick, as early as 18 

 hours after incubation. These are the rudimentary parts of the em- 

 bryo, one end giving origin to the head, and the other corresponding 

 with the tail. The position of these rudiments is remarkably constant 

 in the hen's egg, always lying transversely to the long axis of the egg ; 

 as the embryo-chick develops, it turns upon its side, so that the fore- 

 part of the head usually faces the narrow end of the egg. 



In the rudimentary stage just described, a vertical section across the 

 embryonal line of the germinal area would show the edges of the three 

 germinal layers of the blastoderm, with the primitive groove or furrow 

 in the centre, and the cross-section of the chorda dorsalis beneath it. 



From these three layers, the parts of the future embryo are thus 

 evolved. From the upper external or serous layer, also named the 

 sensorial layer, are developed, along its axial portion, the cerebro-spi- 

 nal 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 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 metamorpho- 

 ses of its substance, the bones, the muscular system, the peripheral 

 spinal nerves, the sympathetic nerves, the heart, bloodvessels, 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 submucous coats of the alimentary 

 canal. Lastly, from the internal layer, also called the mucous or in- 

 testinal layer, are developed the epithelial lining of the alimentary 

 canal, and all its glandular extensions, such as the mucous, gastric, 

 and intestinal glands, the pancreas and the liver, also the lungs and 

 respiratory passages, and the urinary apparatus, including the bladder, 

 ureters, and kidneys. 



Whilst, therefore, the middle layer gives rise, by very striking dif- 

 ferentiations, to a great variety of tissues, the upper and lower layers, 

 except that part of the former which gives origin to the brain and 

 spinal cord, produce textures composed of simpler forms of cell-tissue. 



These layers also contribute, in a manner to be presently described, 

 to the formation of three parts or appendages external to the body of 

 the embryo viz., the amnion, the yolk-sac, or umbilical vesicle, and 

 the allantois. 



GENERAL DEVELOPMENT OF THE EMBRYO AND ITS APPENDAGES. 



The borders of the primitive or vertebral groove, including parts of 

 the external and middle layer (the former named the medullary plate, 

 and the latter the vertebral plate], rise up on each side, and ultimately 



