CELL-LINEAGE. 4 1 



Second, the phenomena we have considered seem to leave 

 no escape from the acceptance of ancestral reminiscence in 

 cleavage, with all that that implies. That the rudimentary 

 entoblasts of Aricia or Spio are such reminiscences of former 

 conditions seems almost as clear as that the mammalian yolk- 

 sac or the avian primitive streak are such. The formation of 

 the ectomesoblast in annelids and mollusks is nearly if not 

 quite as strong a case. Both these are processes that appear 

 to be vestigial, or, at any rate, approach that character. But the 

 evidence of genetic affinity is no less clearly shown in proc- 

 esses that are not vestigial, such as the formation of the ecto- 

 blast in Turbellaria, annelids, and gasteropods or lamellibranchs, 

 from neither more nor less than three quartets of micromeres, 

 or in the origin of the archenteron from the fourth quartet 

 with the remains of the basal quadrants. Between the anne- 

 lids, gasteropods, and lamellibranchs a far more precise and ex- 

 tended series of resemblances exists. The question has been 

 much discussed of late whether such resemblances can be called 

 homologies. Probably no one will deny that the ectoblast-cap, 

 arising from twelve cells, is as a whole homologous in the an- 

 nelid and the gasteropod embryo. Are the individual micro- 

 meres respectively homologous.^ In the present state of our 

 knowledge this is a question of name rather than of fact ; for 

 homologies only gradually emerge during development from 

 their unknown background in the egg. It is for this reason 

 that, as I have urged in a preceding lecture, the ultimate court 

 of appeal in this question lies in the fate of the cells. If the 

 structures to which they give rise are homologous, I can find 

 no logical ground for refusing the claim to the cells from 

 which they arise. Furthermore, this homology must be irre- 

 spective of the origin of the cells, just as the ganglion of a bud- 

 embryo of Botryllus is homologous with that of an egg-embryo 

 in the same form, despite the total difference of origin in the 

 two cases. When, however, we find that the homologous pro- 

 toblasts or parent-cells have the same origin as well as the 

 same fate, the homology becomes the more striking ; and it is 

 in the determination of common origin as well as common fate, 

 as has been done in so many cases, that the principal signifi- 



