No. 3-] THE EMBRYOLOGY OF A TERMITE. 545 



up together that a certain homogeneous reciprocal relation is 

 attained. Now, when certain cells of this layer become altered 

 in nature under the influence of some special forces, the recipro- 

 cation with neighboring cells is likewise altered, resulting in so 

 changing the relations with these latter that a separation of the 

 changed cells from the layer of similar, unchanged cells must 

 take place. 



The mechanical basis of the theory is what happens when a 

 foreign, inorganic particle is introduced into a fluid or viscid 

 layer (" Haut ") whose elemental drops cooperate reciprocally 

 to form a uniform sheet. The foreign particle would be thrown 

 out as a result of purely mechanical tensions. 



So in the case of the origin of the germ-layers, by invagina- 

 tion or immigration ; the sub-epithelial muscle cells of Medusae ; 

 gland cells ; central nervous-system ; sense organs ; Cestode 

 head in the cysticercus ; the extra-embryonal and embryonic 

 cells ; embryonic membranes ; imaginal discs, etc. Whenever 

 two kinds of cells occur in an epithelial layer, one sort is 

 thrown out, so to speak, by invagination or immigration. Cases 

 where this has not taken place represent the early stages of 

 the process (as certain epithelial gland cells or muscle cells). 

 In all these cases the common and necessary cause of invagina- 

 tion and immigration is claimed to be the sharp differentiation 

 of certain cells, physically and chemically, to such an extent 

 that they must move from their primitive position. 



It seems hardly necessary to observe that, though this theory 

 is strictly logical and far-reaching, Wagner does not explain 

 the fundamental question why certain cells rather than all are 

 modified ; and that he overlooks an important and essential 

 difference between the living modified cell in the uniform cell 

 layer, on the one hand, and the foreign, inorganic, dead par- 

 ticle in the homogeneous fluid layer on the other. The theory 

 must collapse when we reflect that, instead of being obliged by 

 the supposed necessity to immigrate as an inorganic particle, 

 the modified living cell could accommodate itself to its old 

 neighbors and remain with them. Of course the inorganic 

 particle would have no such power of adaptation resident in liv- 

 ing protoplasm. Possibly this adaptability of living substance 



