No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 277 



The following chapters are devoted to a comparison of these 

 examples of the alternating type of cleavage from the point of 

 view of (i) homology of cells, (2) developmental mec/ianics, and 

 (3) axial relationships of the embryos. 



I. Homology of Cleavage Cells. 



The importance of the similarity in the cleavage of certain 

 annelids, molluscs, etc., has not been unduly emphasized even by 

 those who have worked on these forms, and has been decidedly 

 underestimated by those who have based their interpretation of 

 cleavage upon the echinoderm and the vertebrate Q.gg. Since 

 my own observations tend to enhance the significance of these 

 similarities, I have been led to make the homology of cells a 

 standpoint for comparison. 



Essential similarity in origin and fate will be considered a 

 sufficient criterion of the homology of cells, as it is, by common 

 consent, of tissues and organs. That the cleavage cells in the 

 forms under consideration are similar in origin is obvious, and 

 is implied by the fact that they constitute a well-defined type 

 of cleavage; but whether they are alike in destiny is more diffi- 

 cult to ascertain. 



The products of the two divisions of the oocytes — the polar 

 globules — correspond in origin and destiny. The cleavage 

 proper may be " equal " or " unequal." 



a. Equal cleavage. 



Hatschek followed the equal cleavage of Eupomattis in detail 

 to sixteen cells, and Wilson describes that of the related 

 Hydroides to thirty cells. The observations on Lepidonotiis 

 recorded in the previous chapters agree precisely with these, 

 and have been extended to the later cleavage. 



The first cleavage furrow is always vertical, but in unequal 

 cleavage the two resulting cells vary more or less in size. 

 The larger size of one product is a means of orientation, 

 and since in all cases it possesses certain potential pecu- 

 liarities, it may be considered in a general way homologous 



