No. I.] THE EMBRYOLOGY OF CREPIDULA. 195 



of ectoblast cells as in Unio, differing, however, in this regard 

 that it is found in quadrants A, B, and C, whereas in Unio it 

 is found in quadrant A only; (14) to this list of accurate resem- 

 blances in the cleavage cells may be added the fact that among 

 annelids and fnollusks the axial relations of all the blastomeres 

 {except possibly the four macromeres) are the same. 



What a wonderful parallel is this between animals so unlike 

 in their end stages ! How can such resemblances be explained ? 

 Are they merely the result of such mechanical principles as 

 surface tension, alternation of cleavage, etc., or do they have 

 some common cause in the fundamental structure of the proto- 

 plasm itself.'' Driesch answers ('92) : " The striking similarity 

 between the types of cleavage of polyclades, gasteropods, and 

 annelids does not appear startling; it is easy to understand this, 

 since cleavage is of no systematic worth." ^ To this, I think, 

 it need only be said in reply that if these minute and long- 

 continued resemblances are of no systematic worth, and are 

 merely the result of extrinsic causes, as is implied, then there 

 are no resemblances between either embryos or adults that may 

 not be so explained. And conversely, these resemblances in 

 cleavage, however they have been produced, stand upon the 

 same basis with adult homologies. 



Within the group of the annelids Wilson ('92) says that 

 "adult homologies are represented by accurate cell homologies 

 in the cleavage stages." But in his general interpretation of 



1 The entire passage (Driesch ('92), p. 41) reads as follows: " Es sind also 

 gewisse aussere Umstande, welche die Furchung beherschen, in Form enipirischer 

 Gesetze ganz oder nahzu bekannt. Wir konnen daraus immerhin Manches lernen, 

 so wird uns die auffallende Ahnlichkeit, welche die Furchungstypen von Poly- 

 claden (Selenka, Lang), Gasteropoden (Rabl, Blochmann, Fol, etc.), und Anneliden 

 (Wilson) darbieten, nicht so sehr frappiren; wir haben eine leises Vej-stdndniss 

 dafiir gewonnen, %oesshalb Furchungsbilder nicht systematisch verwerthbar sind." 



In similar vein he affirms elsewhere ('95, p. 416): " Wenn auch nicht durchauSj 

 so sind also doch in sehr wesentlichem Masse der Furchungsbilder mechanisch 

 verstandlich, wofiir auch die Thatsache spricht, dass bei Nereis, bei Polyclade, und 

 bei Gasteropoden nahezu identisch gestaltet sind; das spricht zugleich gegen ihren 

 Werth fiir phylogenetische Abtheilungen." It should be noted that if cleavage is 

 inherited, as Driesch affirms elsewhere in this same paper, and if certain forms of 

 cleavage are characteristic of species, genera, families, and orders, as is unques- 

 tioned, cleavage does have phylogenetic significance, whether that significance can 

 be extended to widely different types, such as the polyclades and the gasteropods,. 

 or not. 



