. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. 246 BIOLOGICAL ^^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 c
. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. 246 BIOLOGICAL ^^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 Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these il
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