414 



INHERITANCE AND 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. But 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. 188. Diagrams illustrating the value of the quartets in a polyclade (Leptoplana) , a lamel- 

 libranch (Unio), and a gasteropod (Crepidula). A. Leptoplana, showing mesoblast-formation 

 in the second quartet. B. Crepidula, showing source of ectomesoblast (from a 2 , b 2 , c 2 } and en- 

 tomesoblast (from quadrant D}. C. Unio, ectomesoblast formed only from a 2 . 



In all the figures the successive quartets are nun.bered with Arabic figures ; ectoblast unshaded, 

 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 relation to the remainder of the mass, 

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



