H4 BIOLOGICAL LECTURES. 



capitulation theory cannot deny that the embryological evidence 

 is often of the clearest and most convincing character. What 

 is needed is a more trustworthy basis of interpretation ; and 

 until this has been established the embryological method must 

 be employed with the greatest caution. At the present time 

 we do not apparently possess the data necessary to establish 

 such a basis, but certain principles are becoming evident and 

 some of these I shall endeavor to consider. 



The very statement that homologous parts differ in embryo- 

 logical origin itself implies some higher standard of homology 

 that outweighs that of development. What is that standard ? 

 Obviously it is the standard of Owen, viz., the structure and 

 structural relations of the developed organ ; it is the standard 

 of comparative anatomy. It is this criterion that we employ, 

 for instance, in the identification of the ganglion of the Bo- 

 try llus bud, the stomodaeum of the regenerating LumMcu/us, 

 the primary mesoblast-cells of the polyclade or annelid, the 

 neural cell-cords of the leech or earth-worm, or the posterior 

 body-cavities of Tornaria. In all these cases and they might 

 be indefinitely multiplied it is the prospective and not the 

 retrospective aspect of development that is decisive. This is 

 shown most clearly in the case of the germ-layers and the 

 cleavage-stages. In the latter case embryonic origin and posi- 

 tion are utterly valueless apart from developmental destiny. 

 In all these cases homology is determined not by origin, but 

 by fate. And thus we are brought to a point of view directly 

 opposed to that which on the whole is, I believe, the prevalent 

 one to the view, namely, that we must primarily take anatomy 

 as the key to embryology, and not the reverse. Comparative 

 anatomy, not comparative embryology, is the primary standard 

 for the study of homologies, and hence of genealogical descent. 



There are, of course, many special exceptions to this statement, 

 yet I believe on the whole that it is from this point of departure 

 that the renovation and reconstruction of embryological mor- 

 phology must be carried out. The practical bearings of this 

 conclusion cannot be discussed without some consideration of 

 the general nature of development; and the present divergence 

 of opinion on this subject is so great that it will be necessary to 



