188 EMBRYOLOGY. 



tissues take their origin in the separate germ-layers and the mesen- 

 chyme ? A definite answer to this question is possible , except on a 

 few points concerning which the accounts of the different observers 

 are still contradictory, and which therefore will be indicated by a 

 mark of interrogation. 



From the outer germ-layer arise : the epidermis, the epidermoidal 

 organs, such as hair and nails, the epithelial cells of the dermal 

 glands, the whole central nervous system with the spinal ganglia, 

 the peripheral nervous system ('?), the epithelium of the sensory 

 organs (eye, ear, nose), and the lens of the eye. 



The primary inner germ-layer is differentiated into : 



1. The secondary inner germ-layer, or entoblast ; 



2. The middle germ-layers ; 



3. The fundament of the chorda ; 



4. The germ of the mesenchyme, which forms the intermediate 

 layer. 



The entoblast (Darmdriissnblatt) furnishes the epithelial lining 

 of the whole intestinal canal and its glandular appendages (lung, 

 liver, pancreas), the epithelium of the urinary bladder, and the 

 taste buds. 



The middle germ-layers undergo extremely various metamorphoses 

 after having been differentiated into primitive segments and lateral 

 plates. 



From the primitive segments are derived the striated, voluntary 

 muscles of the body and a part of the mesenchyme. 



Frcm the lateral plates aiise the epithelium of the pleuroperitoneal 

 cavity ; the epithelium of ovary and testis (primitive ova, mother- 

 cells of the spermatozoa) ; in general, the epithelial components of 

 the sexual glands and their ducts, as well as those of the kidney and 

 ureter ; and finally mesenchymatic tissue. 



The fundament of the chorda becomes the chorda dorsalis, which in 

 the higher Vertebrates is reduced, during later stages of development, 

 to insignificant remnants. 



The mesenchyme-germs, which produce the intermediate layer, un- 

 dergo manifold differentiations, for they spread themselves out in 

 the body between the epithelial components as the intermediate mass. 

 From them are derived; the multiform group of sustentative (con- 

 nective) tissues (mucous tissue, fibrillar connective tissue, cartilage, 

 bone), vessels (?) and blood (?), the lymphoid organs, the smooth, 

 involuntary muscles of the vessels, of the intestine, and of various 

 other organs. 



