HULL DISCUSSION OK TIM-: PEIXCIPLE8 "F DEVELOPMENT. </ 



The operation of an t growth of the aorfaoe produces q 



another result. When in the middle of a membrane the c 11s of a 

 single- group within a short time repeatedly undergo % 1-y 



vertical plant's, they will l.e suddenly compelled to claim for themselves 



much greater surface, and t h --y will consequently exert a \i;_ r 

 pn -sure, due to growth, upon the cells in their vicinity, and will 

 tend to push them apart. But in this ea>e a separation of contiguous 

 cells such as takes place with gradual and uniformly distril 

 inters; it ial growth, will bo impossible; for the surrounding < 

 remaining in a passive condition, will constitute, as it \v<-re, a rigid 

 frame, as His has expressed it. around the extending part, which, in 

 <|iunce of accelerated growth, demands an incr< . u It 



must therefore secure room for itself in another manner, and inn 

 its surface by abandoning the level of the passive part through 

 the formation of a fold in either one direction or the other. The 

 fold will 1)0 still further increased, and forced farther from the 

 original level, if the increased activity of the process of cell-di vision 

 in it continues. Thus by means of unequal growth there has now 

 arisen out of the originally uniform membrane a new recognisable 

 part, or a special organ. 



When the folding membrane encloses a cavity, as is the case with 

 the blastula. there are /// OO969 OOnC&Vabl* in /// /'nf /'<>,/ ,,/ ; 

 In the lir>t place, the membrane may be folded into the interior of 

 the body, a process which in embryology is called invagination or 

 involution. Secondly, there may arise by evagination a fold, which 

 projects free beyond the surface of the body. 



In the jir*t case numerous variations in the details are possible, so 

 that the most various organs, as, e.g., the glands of the animal body, 

 pivts of the sen.M.ry organs, the central net tern, etc., are 



formed 



In tlf "r'l'i'nt >/ ,//,!/* a small cireumvTil>. d circular part of a 



cellular membrane is infolded as a hollow eytindi ' l and 4 ), 



toward> the interior of the body, into the underlying tissue, and by 

 continuous growth may attain con>ideralle length. The invagina- 

 tion develops into either the tubular or the alveolar form of gland 

 (FLKMMIN'.). If the glandular mouth ; 



blind end nearly uniform dimension^, \ve have the simple tubular 

 gland (tig. 39 1 ), the sweat glands of the skin. I.II:I;I:I;KI UN'S glands 

 of the intestine. The alveolar form of gland ditl'eis from this in that 

 the invaginatd sac doe> nt -in. ply increase in length, but expand - 

 somewhat at its erd (fig. 39 5 , db), while the other part reu 



