246 



BIOLOGICAL LECTURES. 



^^dp' 



Fig. 13. — Arenicola. Somatic plate, dp''- and 

 dp''', cells resulting from division of dp ; at the 

 top of cut the four " intermediate girdle cells " 

 which are formed in front of the prototroch, but 

 come to lie behind it. 



is not merely determinate, as Conklin has defined the term, 

 but the various processes of cleavage are so arranged that the 

 interaction of the component parts leads directly to the estab- 

 lishment of form. The factors of morphogenesis in cleavage 

 are direction and time of division and size of the products, 



accompanied, of course, by 

 more or less differentiation, 

 according as morphogenesis 

 proceeds more or less rapidly; 

 but differentiation, as I be- 

 lieve, is a process wholly dis- 

 tinct from cleavage, whether 

 cleavage be indeterminate, de- 

 terminate, or morphogenetic. 

 Morphogenetic cleavage, of 

 course, possesses in general a 

 real phylogenetic significance. 

 A sufficient cause for the 

 establishment of this form of 

 cleavage is, I believe, the 

 saving of energy which must 

 occur. If the energy of cell 

 division can be utilized to 

 bring material to the point 

 where it is needed, and at the 

 time when it is needed, it is 

 evident that less energy will 

 be required than if extensive 

 migrations were necessary 

 after the formation of the cells. 

 A study in detail of the growth 

 and concrescence of the somatic plate in Arenicola (Figs. 9-18), 

 or in any other form in which it has been fully worked out, 

 affords one of the most beautiful illustrations of the perfect 

 coordination in morphogenetic cleavage, and I think that no 

 one who follows it can doubt that each and every cell division 

 plays its part in bringing about the concrescence with the least 

 expenditure of energy. 



Fig. 14. — Arenicola. Somatic plate, 

 cells of paratroch formed. 



Dorsal 



