No. 1.] THE EMBRYOLOGY OF THE UNIONIDAE. 37 



It is thus possible to speak of an homology of cells. Does 

 it therefore follow that all cells of the same lineage in these 

 different forms are homologous and their products likewise, 

 and that we must deny the homology of cells which, although of 

 different lineage, yet in the end produce homologous organs } 

 I think not. Yet it may be said that if we can speak of ho- 

 mology in the one case, why not in the other } To which it may 

 be answered, that the only safe test of homology in such cases 

 is the end result. We have these facts : (i) Cells of the same 

 lineage have the same fate in a wide series of forms (e.g.y meso- 

 blasts = fourth cleavage of D. (C/". also supra.) (2) Cells of 

 the same lineage have a different fate in forms which otherwise 

 agree closely {e.g.a^-^ in Nereis and Unio), (3) Cells of differ- 

 ent origin have a different fate (numberless instances). (4) 

 Cells of different origin have the same fate, e.g., mesoblast in 

 polyclads and annelids {cf. tables 5 and 7 in appendix), and the 

 protoblasts of the prototroch and velum in annelids and molluscs 

 respectively. Now what are we to say of such a series of con- 

 tradictions .^ 



Simply that it is impossible, in the present state of our 

 knowledge, to explain them all. But this much may be said : 

 The most striking feature is not the contradictions existing, 

 but the wonderful agreements. The first and third of the 

 facts enumerated state the rule ; the second and fourth, the 

 exceptions. The second fact, which is the first exception to 

 the rule, may be explained satisfactorily, as I believe: The 

 ovum, in any stage of development, is an organism complete 

 in itself. Imagine that in any species a new organ is added, 

 or rather, that a diffuse series of structures gains great impor- 

 tance and compactness in the course of evolution. Then, this 

 new structure 7nay be represented in ontogeny by a cell. But 

 the form of cleavage is already defined, and each cell has its 

 allotted destiny. The manufacture of a new cell being an 

 impossibility, an old cell must be modified to represent the 

 new organ. In other species, however, the same cell retains 

 its original functions. This hypothetical case is similar, as I 

 shall attempt to show, to the actual case cited above {vi2.^ a^^ 

 in Nereis and Unio). 



