42 BIOLOGICAL LECTURES. 



cance of recent work in cell-lineage seems to me to lie. Some 

 of the objections urged against the reality of cell-homology 

 have, I think, arisen through a failure to recognize among cell- 

 homologies the same distinction between complete and incom- 

 plete homology that was long ago urged by Gegenbaur in the 

 case of organ-homologies. The posterior member of the fourth 

 quartet in annelids, for example, is in a broad sense homolo- 

 gous throughout the group ; but the homology is probably not 

 an absolute or complete one, since this cell may contain func- 

 tional entoblast {Nereis), rudimentary or vestigial entoblast 

 [Aricia), or apparently in some cases no entoblast, as I have 

 described in Polymnia. Again, the acceptance of cell-homology 

 does not, I think, carry with it the necessity of finding a homo- 

 logue for every individual cell throughout the ontogeny ; for 

 in the case of later structures no one demands or expects that, 

 in the comparison of related forms, an exact equivalent shall be 

 found for every subdivision of homologous nerves or blood- 

 vessels or sense organs. Finally, the fact that cleavage may 

 show no constant or definite relation to the adult parts — as is 

 the case in the teleost fishes- — does not alter the equally in- 

 dubitable fact that cleavage often does show such a constant 

 relation. The probability that the Nanplius larva is not a true 

 ancestral form does not come into collision with the probability 

 that the ascidian tadpole is such a form. How far in the 

 course of phylogeny the ontogeny has adhered to its original 

 type and retained the same relation to the adult parts is a ques- 

 tion which stands, as far as I can see, both a priori and a pos- 

 teriori on essentially the same basis, whether it be applied to 

 the cleavage or to the later stages. Let us not forget the 

 difficulties that still beset us in the application of the biogenetic 

 law to the larval stages and to general organogeny, and let us 

 not make a greater demand in this regard upon cell-lineage 

 than on other lines of embryological research. The time has 

 not yet come for a last word on this subject, and we shall 

 probably have to await the result of much more extended re- 

 search before a satisfactory point of view can be attained. 



