312 THEORIES OF INHERITANCE AND DEVELOPMENT 



areas of differentiation. What gives these cells their specific char- 

 acter ? Why, in the four-cell stage of an annelid egg, should the 

 four cells contribute equally to the formation of the alimentary canal 

 and the cephalic nervous system, while only one of them (the left- 

 hand posterior) gives rise to the nervous system of the trunk-region 

 and to the muscles, connective tissues, and the germ-cells? (Figs. 122, 

 137, B). There cannot be a fixed and necessary relation of cause 

 and effect between the various regions of the ^ZZ which these blas- 

 tomeres represent and the adult parts arising from them ; for, as we 

 have seen, these relations may be artificially altered. A portion of 

 the egg which under normal conditions would give rise to only a 

 fragment of the body will, if split off from the rest, give rise to an 

 entire body of diminished size. What then determines the history 

 of such a portion } What influence moulds it now into an entire 

 body, now into a part of a body ? 



De Vries, in his remarkable essay on Intracellular Pangenesis 

 ('89), endeavoured to cut this Gordian knot by assuming that the 

 character of each cell is determined by pangens that migrate from 

 the nucleus into the cytoplasm, and, there becoming active, set up 

 specific changes and determine the character of the cell, this way 

 or that, according to their nature. But what influence guides the 

 migration of the pangens, and so correlates the operations of devel- 

 opment .^ Both Driesch and Oscar Hertwig have attempted to 

 answer this question, though the first-named author does not commit 

 himself to the pangen hypothesis. These writers have maintained 

 that the particular mode of development in a given region or blasto- 

 mere of the egg is a result of its relation to the remainder of the mass, 

 i.e. a product of what may be called the intra-embryonic environ- 

 ment. Both at first assumed not only that the nuclei are equivalent, 

 but also that the cytoplasmic regions of the Qgg are isotropic, i.e. 

 primarily composed of the same materials and equivalent in struct- 

 ure. Hertwig insisted that the organism develops as a whole as the 

 result of a formative power pervading the entire mass ; that differen- 

 tiation is but an expression of this power acting at particular points ; 

 and that the development of each part is, therefore, dependent on 

 that of the whole. ^ " According to my conception," said Hertwig, 

 "each of the first two blastomeres contains the formative and differ- 

 entiating forces not simply for the production of a half-body, but for 

 the entire organism ; the left blastomere develops into the left half 

 of the body only because it is placed in relation to a right blasto- 

 mere." ^ Again, in a later paper : — " The egg is a specifically 



1 Whitman had strongly urged this view several years before, and a nearly similar concep- 

 tion lay at the bottom of Herbert Spencer's theory of development. Cf. pp. 41, 293. 



2 '92, I, p. 481. 



