THE GERM-LAYERS. 15 



lined with the modified mesodermic elements, known as mesothelium (page 

 21). The cleavage of the middle germ-layer, however, does not involve the 

 mesoderm in the immediate vicinity of the embryonic axis, since on each 

 side of the latter there remains a tract of \\nc\oitparaxial mesoderm, in which 

 appears a series of quadrilateral areas, the mesodermic somites. These are 

 important since they contribute the material giving rise to the vertebral 

 column and the voluntary muscles. 



The parietal layer of the mesoderm adheres to the ectoderm and, in 

 conjunction with the latter, constitutes the somatopleure, the ecto-meso- 

 dermic sheet that forms the ventro-lateral walls of the body. In like man- 

 ner, the visceral layer adheres to the entoderm and, with it, constitutes the 

 splanchnopleare, whose folding-off establishes the digestive tube. 



Since these primary layers give rise to all the tissues of the body, a 

 synopsis of their genetic relations may be given; a word of caution, how- 

 ever, should be added against regarding these groups as too sharply de- 

 fined, since a certain degree of transition must be recognized. 



DERIVATIVES OF THE BLASTODERMIC LAYERS. 



From the ectoderm are derived : 



Epithelium of outer surface of the body, including that of the conjunctiva 

 and anterior surface of the cornea, and of the external auditory 

 canal, together with the epithelial appendages of the skin, as hairs, 

 nails, sebaceous- and sweat-glands (including the involuntary muscle 

 of the latter). 



Epithelium of the nasal fossa, with its glands, as well as the cavities 

 connected therewith. 



Epithelium of the mouth and of the salivary and other glands opening 

 into the oral cavity. 



Enamel of the teeth. 



Tissues of the nervous system. 



The retina ; the crystalline lens and perhaps part of the vitreous humor 

 and the sphincter and dilator of the iris. 



Epithelium lining the membranous labyrinth. 



Epithelium of the pituitary and pineal bodies. 



From the mesoderm, are derived : 



Connective tissues, including areolar tissue, tendon, cartilage, bone and 



dentine. 



Muscular tissue, except that of the sweat-glands and the dilator pupillae. 

 Tissues of the vascular and lymphatic systems, including their endo- 



thelium and circulating cells. 

 All parts of the sexual glands and their excretory passages, as far as the 



termination of the ejaculatory ducts and of the vagina. 

 All parts of the kidney and ureter. 



From the entoderm are derived : 



Epithelium of the digestive tract, with that of all glandular appendages, 

 except those portions of ectodermic origin at the beginning (oral 

 cavity) and termination of the tube. 



Epithelium of the respiratory tract. 



Epithelium of the urinary bladder and of urethra (except part oi male). 



Epithelium of thyroid, parathyroid and thymus bodies. 



