xv] ORGAN-FORMING SUBSTANCES 249 



gated and isolated in particular cells, and in this way the 

 cytoplasm of the different cells comes to be unlike. When 

 once partition walls have been formed between cells the sub- 

 stances in the different cells are permanently separated so 

 that they can no longer commingle 1 ." 



Styela only offers a conspicuous example of a phenomenon 

 that is widespread, for organ-forming substances have been 

 described in the eggs and embryos of animals of very differ- 

 ent groups. Sometimes there appears to be no immediate 

 connection between the substance and the organ or region 

 whose presence it determines. The egg of the Mollusc 

 Dentalium, for example, has a broad equatorial band of 

 reddish pigment, leaving small white anterior and posterior 

 polar areas. In the first cleavage the posterior white area 

 is not divided, but becomes a "yolk-lobe" attached to the 

 two first cleavage cells (PI. XXIV). If before segmentation 

 this white area is cut off, no yolk-lobe is formed, and in the 

 resulting larva not only the post-trochal region, but also the 

 apical tuft of cilia, is absent, so that it would appear that 

 the substance of the yolk-lobe determines the production 

 of parts at opposite ends of the larva (WILSON, 1904). 



Such observations as these, if taken by themselves, would 

 point to the conclusion that the visible substances of the 

 egg are in fact "organ-forming," that is to say, that they 

 are or contain the determinants for the organs with which 

 they are associated. But the experiments of MORGAN (1909, 

 1910), BOVERI (1910), and others in centrifuging eggs that 

 have visible differentiation into regions show that this 

 conclusion is not justifiable in its simple form. MORGAN has 

 centrifuged the eggs of the Frog, the Sea-urchin Arbacia, the 

 Mollusc Cumingia, the Rotifer Hydatina, and other forms, 

 and obtains results in general agreement, indicating that 



1 CONKLIN, 1918, pp. I2J 125. 



