192 CONK LIN. [Vol. XIII. 



of the body from any cell you please." The fact that definite 

 parts of the body come from definite cleavage cells means more 

 than the mere continuity of development, and in this very fact 

 the whole question at issue between determinism and indeter- 

 minism is contained. 



Later work, particularly that of Wilson ('92) on Nereis, 

 Driesch and Morgan ('95) on Ctenophore eggs, and Crampton 

 ('96) on lUyonassa, have led to important modifications of these 

 extreme views. Driesch now sees in cleavage something more 

 than the mere sundering of perfectly homogeneous materials. 

 He grants, what one cannot fail to observe in many cases of 

 determinate cleavage, the existence of cytoplasmic differentia- 

 tions in certain cleavage cells, and even in some cases in the 

 unsegmented &gg (v. Driesch and Morgan ('95), p. 221). He 

 still maintains, however, that the possibility of predicting the 

 prospective significance of single cells is simply a result of the 

 continuity of development as Hertwig's dictum asserts. 



2. Cell and Regional Homologies. 



In looking for the earliest appearing homologies between 

 different animals, embryologists have generally been content 

 to stop with the germ layers. One of the first and most suc- 

 cessful attempts to go back of germ layers was made by Pro- 

 fessor Whitman ('78) in his classical work on the embryology 

 of Clepsine. Since then, under the stimulus of his work and 

 suggestion, there have appeared, chiefly from the Marine Bio- 

 logical Laboratory at Wood's Holl, a remarkable series of con- 

 tributions on this subject of the earliest homologies (cf. Wilson 

 '92 and '93, Lillie '93 and '95, Mead '94, Conklin '92). Owing 

 in large part to the work of this school, there are now sufficient 

 data at hand for making an extensive comparison of every step 

 in the development of a number of annelids, lamellibranchs, 

 and gasteropods. 



(a) Cell Homologies among Annelids and Mollusks. 



Until recently there has been an evident tendency to regard 

 cleavage in different families and orders as exhibiting only 

 general and not detailed resemblances. Thus Bobretzky ('77) 



