128 THEORIES OF HEREDITY 



Weismann sees the causes of individual difference and 

 the divergence of offspring from parent. Weismann 

 believes that the germ-plasm is in reality only found in 

 the nucleus, but the distinction between this and the cell 

 has been omitted from the diagram for the sake of sim- 

 plicity. The germ-plasm is supposed to be represented 

 by the dots in the circle A. 



The second Stage (2) is produced by division of the 

 ovum into two cells (B and C), in one of which (C) a 

 small part of the original germ-plasm, represented by the 

 small circle a, is carried on, unchanged. 



Roux has demonstrated that, if one of the products 

 of the first division of the egg of a frog (B or C in 

 Diagram II) be destroyed with a hot needle, develop- 

 ment is not necessarily arrested, but, when it proceeds, 

 leads to the formation of an embryo from which either 

 the right or the left side is absent. When the first 

 division takes place in another direction, either the 

 hind or front half was absent from the embryo which 

 was afterwards produced. After the next division, 

 when four cells were present, destruction of one pro- 

 duced an embryo from which one-fourth was absent. 1 



The preceding paragraph was written in 1889 before 

 Roux's experiments had been repeated and tested by 

 other workers ; but their arresting interest was such 

 that they soon became the foundation of a prolific school 

 of experimental embryology. The ultimate result of 

 numerous researches is to leave no doubt that Roux 

 was mistaken in some of his conclusions. An excel- 

 lent discussion of the whole question will be found in 

 Professor T. H. Morgan's work. 2 The general con- 

 clusion that there is a necessary correspondence in 

 position between the early embryonic precursors and 

 the organs or parts to which they give rise has been 

 abandoned. On the other hand, the still broader and 

 more fundamental conclusion that definite organ-forming 



1 My attention was first directed to these interesting experiments by 

 Professor Windle's paper in the Journal of Anatomy and PAysiology, 

 vol. xxiii, p. 393. 



2 Regeneration, chapter xi, New York, 1901. 



