344 THE FORMS OF TISSUES [ch. 



egg, we must take brief note of two associated problems: viz. 

 (1) the formation and enlargement of the segmentation cavity, or 

 central interspace around which the cells tend to group themselves 

 in a single layer, and (2) the formation of the gastrula, that is to 

 say (in a typical case) the conversion "by invagination," of the 

 one-layered ball into a two-layered cup. Neither problem is free 

 from difficulty, and all we can do meanwhile is to state them in 

 general terms, introducing some more or less plausible assumptions. 



The former problem is comparatively easy, as regards the 

 tendency of a segmentation cavity to enlarge, when once it has 

 been established. We may then assume that subdivision of the 

 cells is due to the appearance of a new-formed septum within each 

 cell, that this septum has a tendency to shrink under surface 

 tension, and that these changes will be accompanied on the whole 

 by a diminution of surface energy in the system. This being so, 

 it may be shewn that the volume of the divided cells must be less 

 than it was prior to division, or in other words that part of their 

 contents must exude during the process of segmentation*. 

 Accordingly, the case where the segmentation cavity enlarges and 

 the embryo developes into a hollow blastosphere may, under the 

 circumstances, be simply described as the case where that outflow 

 or exudation from the cells of the blastoderm is directed on the 

 whole inwards. 



The physical forces involved in the invagination of the cell- 

 layer to form the gastrula have been repeatedly discussed f, but 

 the true explanation seems as yet to be by no means clear. The 

 case, however, is probably not a very difficult one, provided that 

 we may assume a difference of osmotic pressure at the two poles 

 of the blastosphere, that is to say between the cells which are 

 being differentiated into outer and inner, into epiblast and hypo- 

 blast. It is plain that a blastosphere, or hollow vesicle bounded 

 by a layer of vesicles, is under very different physical conditions 

 from a single, simple vesicle or bubble. The blastosphere has no 

 effective surface tension of its own, such as to exert pressure on 



* Professor Peddie has given me this interesting and important result, but the 

 mathematical reasoning is too lengthy to be set forth here. 



t Cf. Rhumbler, Arch. f. Entw. Mech. xrv, p. 401, 1902; Assheton, ibid, xxxi, 

 pp. 46-78, 1910. 



