GENERAL DISCUSSION OF THE PIUNCIPLES OF DEVELOPMENT. 81 



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

 in the formation of the suture proees.ses 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 indepsndent is subsequently 

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

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

 anterior enlarged portion, which becomes the brain. There are 

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

 which lie in succession one after 

 another; and of these the most an- 

 terior, which becomes the cerebrum 

 with its complicated furrows and con- 

 volutions of first, second, and third 

 order, serves as a classical example 

 when one desires to show how a 

 highly differentiated organ with com- 

 plicated morphological conditions may 

 originate by the simple process of 

 folding. 



In addition to invagination the second 

 method in the formation of folds, 

 which 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 

 protuberances of the surface of the body, which may Kkewise 

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

 of 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 vilh (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 brant hed, so tufted 

 villi are here and there developed out of simple villi, sines local 

 accelerations of growth cause the budding-out of lat. ral branches of 

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

 tufted gUls of various larvse 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 



6 



Fig. 48.— Diagram of the formation of 



papillae and villi, 

 a. Simple paijilla ; b, branched papilla 



or tufted villus ; c, simple papilla, 



the connective-tissue core of which 



runs out into three points. 



