GENERAL DISCUSSION OF THE PRINCIPLES OP DEVELOPMENT. 77 



The operation of an unequal growth of the surface produces quite 

 another result. When in the middle of a membrane the cells of a 

 single group within a short time repeatedly undergo " division " by 

 vertical planes, they will be suddenly compelled to claim for themselves 

 much greater surface, and they will consequently exert a vigorous 

 pressure, due to growth, upon the cells in their vicinity, and will 

 tend to push them apart. But in this case a separation of contiguous 

 cells, such as takes place with gradual and uniformly distributed 

 interstitial growth, will be impossible ; for the surrounding cells, 

 remaining in a passive condition, will constitute, as it were, a rigid 

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

 consequence of accelerated growth, demands an increased area. It 

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

 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 be still further increased, and forced farther from the 

 original level, if the increased activity of the process of cell-division 

 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 two cases conceivable in the formation of folds. 

 In the first 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 first case numerous variations in the details are possible, so 

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

 parts of the sensory organs, the central nervous system, etc., are 

 formed. 



In the origin of glands a small circumscribed circular part of a 

 cellular membrane is infolded as a hollow cylinder (fig. 39 1 and 4 ), 

 towards the interior of the body, into the underlying tissue, and by 

 continuous growth may attain considerable length. The invagina- 

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

 (FLEMMING). If the glandular sac possesses from its mouth to its 

 blind end nearly uniform dimensions, we have the simple tubular 

 gland (fig. 39 l ), the sweat glands of the skin, LIEBERKUHN'S glands 

 of the intestine. The alveolar form of gland differs from this in that 

 the invaginated sac does not simply increase in length, but expands 

 somewhat at its end (fig. 39 5 , db), while the other part remains 



