No. 3.] AMPHIOXUS AND THE MOSAIC THEORY. 609 



larvae. In only a single isolated case did one of these larva 

 develop as far as the latest stage reached by the y^, larva, and 

 this individual showed several defects, {c) The ^ blastomere 

 segments in a form suggesting that of the normal ovum but 

 never identical with it. The gastrula stage is never reached 

 and only in comparatively rare cases is a blastula formed. 

 Most individuals develop approximately as they would if still 

 forming part of an entire embryo and give rise to partial 

 embryos — i.e., those that correspond in a measure to fragments 

 of an entire embryo. The same is true of the 1/16 blastomere. 

 • It might be supposed that these progressive limitations are 

 due simply to the progressive diminution in the size of the 

 individual blastomeres. There are, however, two considerations 

 that seem to render this explanation inadequate. The first is 

 the occasional production of >^ -sized gastrulas from {} frag- 

 ments of) 2-celled or 4-celled stages (p. 589). The second is the 

 progressive change in the/^rw of cleavage (e.g., the inequality 

 of the second cleavage of isolated yi blastomeres), which does 

 not seem explicable as the result either of diminished size 

 or of alteration in contour, since all of the isolated blasto- 

 meres become nearly spherical before their first division. It 

 seems to me, therefore, that this gradual diminution of de- 

 velopmental and regenerative power in the individual blasto- 

 meres as thp cleavage advances, together with their increasing 

 departure from the forms of cleavage shown by the entire 

 ovum, must be due to their progressive differentiation and 

 the relation of this process to regeneration may be conceived 

 as follows, in accordance with Hert wig's general view. As 

 the ontogeny advances the idioplasm of the cells undergoes 

 gradual and progressive physiological modifications (brought 

 about by the interaction of the various parts of the embryo), 

 without, however, anywhere losing any of its elements. The 

 isolation of a blastomere restores it in a measure to the con- 

 dition of the original ovum and the idioplasm, therefore, tends 

 to return to the condition of the original germ-plasm and thus 

 to cause a repetition of the development from the beginning. 

 The manner in which the blastomere responds to this change 

 of environment depends, however, upon two conditions; first, 



