THE ORGANS OF THE OUTER GERM-LAYER. 521 



amniotic fluid and make it turbid. Finally these, as well as some 

 of the detached downy hairs, may be swallowed by the embryo with 

 the amniotic fluid, and thus become a component of the meconium 

 accumulated in the intestine. 



The epidermis constitutes only one component of the skin of the 

 adult or of the integument ; the other and more voluminous part 

 the derma or corium is produced by the mesenchyme. The same thing 

 takes place here as in the case of the other membranes and organs 

 of the body. The epithelial layers derived from the primary germ- 

 layers enter into close relationship with the mesenchyme, since they 

 acquire from the latter a connective-tissue foundation that serves for 

 their mechanical support and nutrition. Just as the inner germ- 

 layer unites with the intermediate layer to form the mucous mem- 

 brane of the alimentary canal, as the epithelium of the auditory 

 vesicle with the adjacent connective substance to form the mem- 

 branous labyrinth, and as the epithelial optic vesicle with the choroid 

 and sclera to form the eyeball, so here also the epidermis unites with 

 the corium to constitute the integument. 



During the first months the corium forms in Man a layer of 

 closely packed, spindle-shaped cells, and is delimited from the 

 epidermis by a delicate, structureless, smooth-surfaced, bounding 

 membrane (basement membrane), such as exists permanently in the 

 case of the lower Vertebrates. In the third month it is differ- 

 entiated into the corium proper and the looser subcutaneous tissue, 

 in which there are soon developed clusters of fat cells. From the 

 middle of pregnancy onward the latter so increase in number that 

 the subcutaneous tissue soon becomes a layer of fat covering the 

 whole body. At this time the smooth contour between epidermis 

 and corium is lost, owing to the development on the surface of the 

 latter of small papillae, which grow into the mucous layer and 

 produce the corpus papillare of the skin. The papillae serve partly 

 for the reception of loops of capillary blood-vessels, and thus effect 

 a better nutrition of the mucous layer ; in part they receive the 

 terminations of tactile nerves (tactile corpuscles), and thus are 

 divided into vascular papillse and nervous papillae. 



The skin of Vertebrates attains a higher degree of development in 

 consequence of processes similar to those described for the intestinal 

 canal. The epidermis increases its surface outward by the formation 

 of folds, inward by invaginations. Because the evaginated and 

 invaginated parts at the same time alter in many ways their 

 histological peculiarities, there arises a large number of organs of 



