GENERAL DISCUSSION OF THE PRINCIPLES OF DEVELOPMENT. 81 



O 



plic 

 folc 



whi 



gin 



par 



fori 



proti 



assui 



of s: 



epidermis, while the other part is annexed to the medullary tube. Thus 

 in the formation of the suttire processes of fusion and of separation 

 occur almost simultaneously, a condition which often recurs in the 

 case of other invaginations, as in the constricting off of the auditory 

 vesicle, the vesicle of the lens, etc. 



The neural tube having once become independent is subsequently 

 segmented in manifold ways by the formation of foldings, in conse- 

 uence of inequalities in the rate of surface growth, especially in its 

 terior enlarged portion, which becomes the brain. There are 

 med out of this by means of .four constrictions five brain-vesicles, 

 hich lie in succession one after 

 other ; and of these the most aii- 

 rior, which becomes the cerebrum 

 ith its complicated furrows and con- 

 lutions of first, second, and third 

 'der, serves as a classical example 

 en one desires to show how a 

 ighly differentiated organ with coni- 

 licated morphological conditions may 

 iginate by the simple process of 

 biding. 



In addition to invagination the second 

 thod in the formation of folds, 

 hich depends upon a process of eva- 

 gination, plays a no less important 

 part in the determination of the 

 form of animal bodies, giving rise to 



rotuberances of the surface of the body, which may likewise 

 me various forms (fig. 42). As a result of exuberant growths 

 small circular territories of a cell-membrane there arise rod- 

 like elevations, resembling the papillae on the mucous membrane 

 of the tongue (c), or the fine villi (a) in the small intestine (villi 

 intestinales), which are so closely set that they give a velvety ap- 

 pearance to the surface of the mucous membrane of the intestine. 

 Just as the tubular glands may be abundantly branc hed, so tufted 

 "illi are here and there developed out of simple villi, sine 3 local 

 elerations of growth cause the budding-out of lateral branches of 

 a second, third, and fourth order (fig. 42 b). We re call the external 

 tufted gills of various larvae of Fishes and Amphibia, which project 

 out from the neck-region free into the water, or the villi of the 

 chorion in Mammals, which are characterised by still more numerous 



Fig. 42. Diagram of the formation of 



papillae and villi. 

 a, Simple papilla ; b, branched papilla 



or tufted villas; c, simple papilla, 



the connective-tissue core of which 



runs out into three points. 



villi 

 ace. 



