414 



INIIERITAXCE .LVD DEVELOPMENT 



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. Hut what influence guides the 

 migrations 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 {Lepfpplana) ,n lamel- 

 libranch {Uiiio), and a gasteropod {Crepidiila). A. Leptoplana, showing mesoblast-formation 

 in the second quartet. B. Crepidiila, showing source of ectomesoblast (from d^, b'^, c'^) and en- 

 tomesoblast (from quadrant D). C. 17?/ w, ectomesoblast formed only from d~. 



In all the figures the successive quart€ts are numbered with Arabic figures ; ectoblast unsliaded, 

 mesoblest dotted, entoblast vertically lined. 



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 jrlation to the remainder of tJie mass, 

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



