Herrick, Physiological Prohleius. 373 



progressive process, for, as he says,^ "in the latter case * * * pre- 

 localization, even in the unsegmented egg, may be brought under 

 the category of epigenetic phenomena (epigenetic qualities as dis- 

 tinguished from preformed qualities), and falls into harmony with 

 hypotheses that assume the nucleus to be the primary determining 

 factor. 



The results of Yatsu on Cerebratulus show that the localization 

 of the morphogenic factors is progressive even in stages preceding 

 cleavage. The percentage of normal larvae obtained from frag- 

 ments steadily diminished with age and localization becomes pro- 

 gressively more definite. Cleavage and morphological factors may 

 or may not develop cotemporaneously. Such prelocalization exists 

 in Dentalium before the egg leaves the ovary, long before initial 

 stages of maturation and fertilization. By separation of the 

 blastomeres several distinct imperfect individuals may be produced, 

 and, except for readjustment of cells, undergo practically the same 

 development as if unseparated, thus proving a mosaic theory. 

 In Dentalium the clear substance forming the polar lobe is visible 

 prior to cleavage and mutilation experiments prior to segmenta- 

 tion show that this substance has practically the same morphogenic 

 value as the polar lobe itself. 



It is evident that, in spite of the recognition given by Wilson to 

 Morgan's theory of morphallaxis in the closing and readjustment 

 of the body of an embryo resulting from the development of a 

 fragment of an egg, the former interprets the phenomena in a 

 mechanical rather than a vitalistic way. The regulative capacity 

 of such embryos is very great, however. The same final result 

 may be reached in the case of fragments of unsegmented eggs and 

 separated blastomeres in different ways showing the identical regu- 

 lative impress of these unlike processes. 



We think a dynamic explanation applicable even upon the basis 

 of prelocalization and a mosaic theory of development. As Wil- 

 son says: "The early development of egg fragments indicates that 

 the specification of the cytoplasmic regions is primarily qualitative 

 but not quantitative, or if quantitative is still subject to a regulative 

 process that lies behind the originial topographical grouping of 

 the egg-materials" (o/*. r/>., p. 70). But qualitative specification 

 must be dynamic; all qualities are the result of behavior. The 

 smallest part of any " Keimbezirk" is capable of regenerating true 



^Journ. Exp. ZooL, VoL i, No. i, p. 2. 



