The cell in development and inheritance . he egg which these blastomeres represent and the adultparts arising from them ; for in some eggs these relations may beartificially changed. A portion of the egg which under normal con-ditions would give rise to only a fragment of the body will, if split offfrom the rest, give rise to an entire body of diminished size. Whatthen determines the history of such a What influencemoulds 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
The cell in development and inheritance . he egg which these blastomeres represent and the adultparts arising from them ; for in some eggs these relations may beartificially changed. A portion of the egg which under normal con-ditions would give rise to only a fragment of the body will, if split offfrom the rest, give rise to an entire body of diminished size. Whatthen determines the history of such a What influencemoulds 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 thecharacter of each cell is determined by pangens that migrate from 414 INHERITANCE AND DEVELOPMENT the nucleus into the cytoplasm, and, there becoming active, set upspecific changes and determine- the character of the cell, this wayor that, according to their nature. But what influence guides themigrations of the pangens, and so correlates the operations of devel-opment? Both Driesch and Oscar Hertwig have attempted to. Fig. i88. — Diagrams illustrating the value of the quartets in a polyclade {Leptoplana), a lamel-libranch {Uiiio), and a gasteropod {Crepidula). A. Leptoplana, showing mesoblast-formationin the second quartet. B. Crepidiila, showing source of ectomesoblast (from a^, 32, c^) and en-tomesoblast (from quadrant D). C. Unio, ectomesoblast formed only from a^. In all the figures the successive quartets are numbered with Arabic figures ; ectoblast unshaded,mesoblest dotted, entoblast vertically lined. answer this question, though the first-named author does not commithimself to the pangen-hypothesis. These writers have maintainedthat the particular mode of development in a given region or blasto-mere of the o-gg is a result of its relation to the remainder of the mass, a product of what may be called the intra-embryonic environ- NATURE AND CAUSES OF DIFFERENTIATION 415 ment. Hertwig insisted that the organism develops as a whole asthe result of
Size: 1526px × 1637px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, booksubjectcells, bookyear1902
