VII] OF THE GASTRULA 561 



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 estab- 

 lished. 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 diminu- 

 tion 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 

 several explanations are conflicting, and are far from clear. There 

 is, however, a certain homely phenomenon which goes some way, 

 perhaps a long way, to explain this remarkable configuration. An 

 ordinary gelatine lozenge, or jujube, has (Uke the developing 

 gastrula) a more or less spherical form, depressed or dimpled at one 

 side; this is a very noteworthy conformation, and it arises, auto- 

 matically, by the shrinkage of a sphere. Were the initial sphere of 

 gelatine perfectly homogeneous, and so situated as to shrink with 

 absolute uniformity, it would merely shrink into a smaller sphere; 

 it does nothing of the kind. There is always some part or other 

 which shrinks a little 'hore than the rest J ; and the dimple so formed 

 goes on increasing, until at last a very perfect cup-shaped figure is 

 formed. I imagine that the gastrula is formed in much the 



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

 reasoning is too lengthy to be set forth here. 



t Cf. Butschli, Arch.f. Entw. Mech. v, p. 592, 1897; Rhumbler, ibid, xiv, p. 401, 

 1902; Assheton, ibid, xxxi, p. 46, 1910. 



X Just as there may be some small part which shrinks a little less. But this 

 we should not distinguish from the common case where one small part grows 

 a little more, and so "produces a 6ud," as in the yeast-cell on p. 363. 



