I q8 conk LIN. [Vol. X 1 1 1 . 



determining whether they have the same structure is to observe 

 what they develop into. If certain embryonic parts always 

 give rise to certain definitive structures, the conclusion is war- 

 ranted that these parts themselves must be alike in structure. 

 The homology of germinal layers, therefore, must have refer- 

 ence to prospective resemblances, and accordingly the test of 

 all such homologies must be the history and destiny of those 

 layers (cf. Wilson, 95). 



If, however, prospective resemblances form a basis for 

 homology, there is no reason for stopping with germ layers in 

 seeking to find the earliest homologies. In those cases in 

 which an entire layer can be reduced to a single cell, how is it 

 possible on morphological grounds to affirm homology of the 

 layer but to deny it to the cell 1 Is it not evident that an 

 altogether unnatural distinction is made when an imaginary 

 line is drawn between blastomeres and layers, on the one side 

 of which homologies may be predicated and on the other not .-' 



If organs which are homologous among annelids and mollusks, 

 such as the prototroch, the apical sense organ, the stomodaeum, 

 and the ventral plate, can be traced back in their development 

 to certain individual cells of similar origin, position, size, and 

 history, are not these cells truly homologous .-' If not, where in 

 this developmental process shall we say that homologies begin ? 



I believe there is no escape from the conclusion that the 

 protoblasts of homologous organs are as certainly homologous 

 as are the organs to which they give rise, that the protoblasts 

 of homologous layers are as surely homologous as are those 

 layers, and that the protoblasts of definite regions are as much 

 homologous as are those regions. We therefore reach the con- 

 clusion that, in related organisms with determinate cleavage, 

 homologies may be predicated of single cells, whether they be 

 protoblasts of the nervous system, the excretory system, or the 

 locomotor apparatus; of the ectoderm, the mesoderm, or the 

 endoderm; of the right or left, the anterior or posterior por- 

 tions of the body. 



It does not matter, so far as the fundamental idea of cell 

 homology is concerned, how such homology may have arisen. 

 The definite character of the cleavage of Nereis is ascribed by 



