162 INTERNAL FACTORS IV. i 



surrounded by masses of pigment and in addition to irregular 

 chromatic masses some pale, some deeply staining. Separating 

 it from the uninjured half is a thin layer of yolkless protoplasm. 



Reorganization is accomplished by any one of three methods. 

 The first is nucleation followed by cellulation, that is to say, 

 masses of cytoplasm arise round normal .nuclei, some of which 

 are derivatives of the injured nucleus, while others have migrated 

 across from the living blastomere. The second method involves 

 an abundant immigration of whole cells from the living embryo 

 which resuscitate or feed on the yolk and nuclei of the injured 

 half. In the third method overgrowths take place at various 

 points of the tissues of the living half-embryo, the cells dipping 

 into and feeding on the yolk as they pass across. 



In post-generation, which only occurs after the first method 

 of reorganization, each layer of the uninjured reforms the corre- 

 sponding layer of the injured, the cells of the former exerting 

 a directive stimulus upon the reorganized tissues of the latter, 

 which thus pass through a process of ( dependent ' differentiation. 

 The ectoderm grows over from in front backwards and from 

 below upwards, so that a structure superficially resembling a 

 yolk-plug is formed at the hinder end; as it does so it becomes 

 differentiated into the usual two layers. The medullary tube is 

 formed in like manner. The mesoderm is formed from the 

 ventral side and later divided into vertebral and lateral plates. 

 The notochord was a whole from the beginning. The missing 

 half of the gut is formed directly from the half-gut of the living 

 embryo, not by any process of blastoporic invagination. 



The conclusions drawn by Roux from this experiment have 

 already been stated. Development is conceived of as a process 

 governed essentially by a factor which is entirely internal, the 

 preformed structure of the nucleus of the germ. Into the 

 validity of this factor we have now to inquire by examining 

 the evidence offered by the very numerous and similar experi- 

 ments, performed on the eggs of animals of all kinds, which the 

 ' Mosaik-Theorie ' has evoked. We shall take these experiments 

 in order, beginning with the form employed by Roux himself, 

 the common Frog l . 



1 For literature see following section. 



