410 PROFESSOR LANKESTER. 



of the connection between Delamination and Invagination. 

 All differentiation of cells, the development of one kind of 

 cell from another kind, is dependent on internal movements 

 of the physiological molecules of the protoplasm of such cells. 

 When Delamination occurs in the cells of the blastula of 

 Geryonia, or when it occurred in the ancestral blastula, the 

 molecules destined to build up the enteric cell and deric cell, 

 into which one of the primitive cells divides, are already 

 present before they are made visible to the eye by segrega- 

 tion and accumulation on opposite faces of the differentiating 

 cell. Though the substance of a cell may appear homo- 

 geneous under the most powerful microscope, excepting for 

 the fine granular matter suspended in it, it is quite possible, 

 indeed certain, that it may contain, aheady formed and 

 individualised, various kinds of physiological molecules. The 

 visible process of segregation is only the sequel of a differen- 

 tiation already established, and not visible. The descendants 

 of the Diblastula (diploblastic Planula) , which had gradually 

 acquired a separate deric and enteric cell-layer in place of 

 one cell-layer with an external deric moiety and an internal 

 enteric moiety to each cell, must have tended in their indi- 

 vidual development from the egg- cells of parent Diblastulae 

 to have established more and more early, in the course of 

 their growth, the important separation of deric and enteric 

 cells, of ectodermic and endodermic elements. In so far as 

 the differentiation of the two kinds of factors or molecules, 

 the deric and the enteric, became dependent on heredity, 

 and less dependent on the direct adaptative causes which 

 first brought about the differentiation, in so far would it be 

 possible for the differentiation, the segregation of deric 

 molecules from enteric molecules, to take place at an earlier 

 point in the embryonic development than that (namely, the 

 blastula stage), at which the direct adaptative causes coidd 

 come into operation. Thus, since the fertilised egg already con- 

 tained hereditarily acquired molecules, both deric and enteric, 

 invisible though difl'erentiated, there would be a possibility that 

 these two kinds of molecules should part company, not after 

 the egg-cell had broken up into many cells as a morula, but at 

 the very first step in the multiplication of the egg-cell. In 

 fact, some or all of the deric molecules might remain in one 

 of the two first cleavage-cells, and all of the enteric mole- 

 cules, with or without some of the deric molecules, might 

 remain in the other. We should not be able to recognise 

 these molecules by sight; the two cleavage-cells would 

 present an identical appearance, and yet the segregation of 

 deric and enteric factors had already taken place. This 



