CELL-LINEAGE. 23 



If we survey the general field of embryology, we find that 

 ancestral reminiscence in development is most conspicuously 

 shown and has been longest known in the later stages, and 

 many of the most interesting and hotly contested controversies 

 of modern embryology have been waged in the discussion of 

 the possible ancestral significance of larval forms, such as the 

 trochophore, the Nauplius, the ascidian tadpole, and many 

 others. It is generally admitted, too, that ancestral remi- 

 niscences may occur in earlier embryonic stages. While few 

 naturalists would to-day accept Haeckel's celebrated Gastraea 

 theory in its original form, probably still fewer would deny that 

 the diblastic embryo (gastrula, planula, etc.) of higher forms 

 is in a certain sense reminiscent of the origin of these forms 

 from diblastic ancestors having something in common with 

 existing ccelenterates. 



It is in respect to still earlier stages, namely, those including 

 the cleavage of the egg, that the greatest doubt now exists ; 

 and there is hardly a question in embryology more interesting 

 or more momentous than whether these stages may exhibit 

 ancestral reminiscence, and whether they, like the later stages, 

 exhibit definite homologies, and thus afford in some measure a 

 guide to relationship. None of the earlier embryologists were 

 disposed to answer this question in the affirmative. To them, 

 and it should be added to some of our contemporaries as well, 

 the cleavage of the ovum was " a mere vegetative repetition of 

 parts," the details of which had no ancestral significance, and 

 the ontogeny first acquired a definite phyletic meaning and in- 

 terest with the differentiation of the embryonic tissues and 

 organs. To these observers the cleavage of the ovum pre- 

 sented merely a series of problems in the mechanics of cell- 

 division, and its accurate study was almost wholly neglected as 

 having no interest for the historical study of descent. And 

 yet it was long ago shown that the blastomeres of the cleaving 

 ovum have in some cases as definite a morphological value as 

 the organs that appear in later stages. Kowalevsky and Rabl 

 traced the mesoblast-bands in annelids and gasteropods back to 

 a single cell, which still later research has shown to have the 

 same origin and fate, and hence to be homologous in the two 



