A minute and accurate study of the early stages of division or cleavage of the egg of annelids has shown a remarkable agreement throughout the group. The work of E. B. Wilson on nereis, and on a number of other forms, as well as the subsequent work of Mead, Child, and Treadwell on other annelids, has shown resemblances in a large number of details, involving some very complicated processes.[10] Footnote 10: On the other hand it should not pass unnoticed that Eisigh as shown in one form (in which, however, the eggs are under special conditions being closely packed together) that the usual type of cleavage is altered. Not only is the same method of cleavage found in most annelids, but the same identical form of division is also present in many of the mollusks, as shown especially by the work of Conklin, Lillie, and Holmes. This resemblance has been discussed at some length by those who have worked out these results in the two groups. The general conclusion reached by them is that the only possible interpretation of the phenomenon is that some sort of genetic connection must exist between the different forms; and while not explicitly stated, yet there is not much doubt that some at least of these authors have had in mind the view that the annelids and mollusks are descended from common ancestors whose eggs segmented as do those of most of the mollusks and annelids of the present day. This conclusion is, I believe, of more far-reaching importance than has been supposed, and may furnish the key that will unlock the whole question of the resemblance of embryos to supposed ancestral forms. It is a most fortunate circumstance that in the case of this cell lineage the facts are of such a kind as to preclude the possibility that the stages in common could ever have been ancestral adult stages. If this be granted then only two interpretations are possible: the results are due either to a coincidence, or to a common embryonic form that is repeated in the embryo of many of the descendants. That the similarity is not due to a coincidence is made probable from the number and the complexities of the cleavage stages.