OKNKHAI. 1>1S< 1 SSloN } I III! PE1N< [PI E8 OF ] 81 



rpMermis, \\ bile tbe M her part is annexed to th.- medullary tube. | 

 in the ftn'inntitm if the suture processes of fu>in and of -,.p ; ,i 

 occur almo>t simultaneously, a condition which often n cur> in the 

 of other invaginat ions, as in the constricting oil' of the auditory 



V. side, the V. side of tbe lens, e(c. 



'Hi.- neural tube having once become indep ndent is >ubse(piently 

 segmtnted in manifold way> by t be formation of foldings, in conse- 

 (ju. nee of in qualities in tbe rate <,f 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 v. >icl s, 

 which lie in succession one after 

 another; and of these the most an- 

 ttrior, which Incomes the cerebrum 

 with its complicated fui rows and con- 

 volutions of first, second, and tbiid 

 order, serves as a classical example 

 win n 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 invaginat ion the second 

 uf//<f i/t t/ie for/nation of y'eA/.v, 

 which depends upon a process of era- 

 filiation, plays a no less imp* 

 part in the determination of tin- 

 form of animal bodies, giving rise to 

 protuberances of the surface of the body, which may \\k< 

 a.-sume various forms (fig. 42). A> a result of exuberant growths 

 -mall circular ttrntorns of a (vll-meinbrane there arise rod- 

 like Irvations, resembling the papilla* on the mucous mem! 

 of the tongue (c), or the tine \illi (n) in the small inUstine (villi 

 int. stinahs). which are so closely set that they give a velvety ap- 

 parai:ce to the surface of the mucous membrane of the int, 

 Just as the tubular glands may be abundantly brain b. d. M> tufted 

 villi are here and there d. velopi d out of >imple villi. >inr local 

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

 a second, third, and fourth order (fig. 42 6). We r. call tbe ..xt.rnal 

 tufted gills of various larvn- of Ki^h, < ai.d Amphibia, which project 

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

 chorion in Mammals, which are characterised by still more numerous 



6 



Fig. 42. Diagram of the formation of 

 papilla and villi. 



If ii.ill;i ; f>, bran.-lu.l |-apill.i 

 or tufted villas; c, 8ini}>l? |..q iln. 

 the connective-tiasue core of whuh 

 runs out into three points. 



