202 CONKLIN. [Vol. XIII. 



tween the two. Polar differentiation, however produced, seems 

 to be essentially the same in all cases.^ In very many bilateral 

 animals the animal pole forms the cephalic end of the antero- 

 posterior axis, though it frequently undergoes great shiftings to 

 reach that point. So far as known, the trunk region of annelids 

 and mollusks always comes from the same region of the em- 

 bryo. The prototroch always comes from the region between 

 the first and second quartettes of ectomeres. There is good 

 reason to believe that in all cases each quartette of ectomeres 

 occupies homologous regions in the adult. In all these cases 

 there are fundamental homologies of regions, though homolo- 

 gous parts may not always be limited by homologous cell 

 walls, as Whitman ('94) has argued. The fact, however, that 

 so many accurate cell homologies exist among several different 

 groups seems to me to indicate that the formation of cells 

 has a more important role in development than Whitman 

 assigns to it (cf. Wilson, '94). 



Conclusions. 



In general, the forms of cleavage are the result of three 

 distinct classes of factors, which may vary in importance in 

 different animals, (i) The first and simplest of all are the 

 mechanical conditions, such as surface tension, alternation of 

 cleavage, and the like. These conditions are always present 

 and are generally, though not invariably, fulfilled, the result 

 being that certain fundamental features of all cleavage may 

 be referred to such factors. 



(2) The fact that cleavage is an inherited character and that 

 definite forms of cleavage and accurate cell homologies are 

 characteristic of several great groups of animals gives it a cer- 

 tain phylogenetic value, for however they may have been pro- 

 duced, inherited structural likenesses which run through closely 

 related species, genera, orders, and types must be considered to 

 have a phylogenetic value. The fact that such likenesses are 

 real homologies, as has been argued elsewhere, is evidence 

 upon this point. 



1 One remarkable exception to this statement is known. Castle ('96) has 

 found that the polar bodies are formed at the vegetal (entoderm) pole in Ciona, 

 and the same is probably true of other ascidians. 



