No. I.] THE EMBRYOLOGY OF CREPIDULA. 197 



been done. It has been generally conceded that the trocho- 

 phore larva which appears in the development of both of these 

 groups is evidence of their former connection, but the resem- 

 blances mentioned above show that in the prelarval stages, and 

 also in the metamorphosis following the trochophore stage, 

 there are many resemblances between the two groups, particu- 

 larly in the history of the somatoblasts, the formation of the 

 trunk, and the establishment of bilateral symmetry. 



On the other hand, the embryological history only serves to 

 widen the gap between the cephalopods and other mollusks, 

 for in the early development there is apparently nothing in 

 common between the two. 



The application of the word homology to pregastrular stages 

 may deserve a short explanation and justification. This term 

 as employed by Owen was used to denote morphological corre- 

 spondence in the relative structure, position, and connection of 

 adult parts; but since this morphological correspondence is 

 characteristic of the parts of embryos as well as of adults, it is 

 evident that to rigidly limit the word homology to adult char- 

 acters would be to draw a wholly artificial and useless distinc- 

 tion between adult and embryonic structures. Accordingly, we 

 find that the word has been very generally used to denote 

 morphological correspondence of embryonic, as well as of adult, 

 parts, and this correspondence was found in earlier and earlier 

 stages of the ontogeny, until Huxley finally homologized the 

 germinal layers of higher metazoa with the cell layers of adult 

 coelenterates. 



The chief objections which have been raised in recent years 

 against the general homology of the germ layers arise from the 

 fact that the layers in themselves have been regarded as organs 

 which might be compared as if they were adult parts. They were 

 estimated by what they were rather than by what they might 

 become, and consequently false ideas often obtained as to what 

 they really were; for the real structure of embryonic parts can 

 usually be determined only by observing the entire history of 

 those parts. I presume no one supposes that we can directly 

 recognize and compare the fundamental structure of eggs, 

 blastomeres, or layers; at present the only satisfactory way of 



