38 BIOLOGICAL LECTURES. 



tina. The differences which Wilson points out between these 

 two structures do not, therefore, exist. In both annelids and 

 mollusks the prototroch lies at the boundary between the first 

 quartette on one side and the second and third on the other. 

 In both there is found a preoral, an adoral, and a post-oral band 

 of cilia; (9) in the gasteropod the apical cells give rise to an 

 apical sense organ such as is found in many annelid trocho- 

 phores; (10) the supra-oesophageal ganglia and commissure 

 apparently arise from the same group of cells in annelids and 

 gasteropods; (n) the fourth quartette in annelids and gastero- 

 pods contains mesoblast in quadrant D, but is purely entoblastic 

 in quadrants A, B, and C; (12) a fifth quartette is formed in 

 gasteropods and some annelids (Amphitrite, etc.), and consists 

 of entoblast only; (13) in the gasteropod larval mesoblast arises 

 from the same group of ectoblast cells as in Unio, differing, how- 

 ever, 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 resemblances in the cleavage cells may be 

 added the fact that among annelids and mollusks the axial 

 relations of all the blastomeres (except possibly the four macro- 

 meres] 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 : ' The striking similarity be- 

 tween 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 resem- 

 blances in cleavage, however they have been produced, stand 

 upon the same basis with adult homologies." J 



1 "Embryology of Crepidula,"_/<9z/r720/ of Morphology, XIII, No. i. 



