THE SPIRAL TYPE OF CLEAVAGE. 



247 



Fig. 15. — Arenicola. 

 matic plate, showing position of para- 

 troch when first formed, x x, the 

 first cells which meet in the median 

 line during concrescence. 



Since every cell plays a definite part, 

 it is evident that morphogenetic cleav- 

 age is necessarily accompanied by a 

 certain degree of segregation, i.e., the 

 material which is to form the various 

 organs is more or less completely 

 segregated into definite cells or groups 

 of cells, though they may not be dif- 

 ferentiated, or, rather, the differentia- 

 tion may be simply one of position. 



It appears probable that this bilat- 

 erally symmetrical, morphogenetic 

 cleavage has encroached upon the 

 spiral period during the course of 

 phylogenetic development and short- 

 ened it, and that, in addition, inde- 

 pendent modifications have arisen 

 in the different groups, thus altering 

 the relations of the constituent ele- 

 ments. A study of the differences 

 in cleavage in various forms brings to 

 light a number of facts which appear 

 to justify these conclusions. 



In the following paragraphs I have 

 brought together some of the facts 

 which appear to me to furnish evidence 

 upon these points. 



I. The spiral period proper ends 



with different cell generations in dxi-YiG.-LT. — Arenkola. Portion of so- 

 matic plate. The cells -r j: in contact 

 f erent forms. in median line. 



Meager as are the data for a comparison 

 between the different larger groups, they 

 indicate what I believe will be amply 

 demonstrated in the future, viz., that as 

 the process of development becomes viore 



and viore highly modified, the strictly spiral ^^—Arenkoia Para- 



cleavage, when originally presetit, gives zvay troch just before ventral ends 



_ _ are brought into contact by the 



at aji increasingly early stage to a tnorpho- concrescence of somatic plate. 



Fig. 16. — Arenicoia. Portion of somatic 

 plate. Concrescence proceeding. 



