No. 3-] AMPHIOXUS AND THE MOSAIC THEORY. 6 1 



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and the adult structures to which they give rise is a purely 

 casual one. [" In Folge der Kontinuitat der Entwicklung muss 

 ja natiirlicherweise jede altere Zellengruppe sich auf eine 

 vorausgegangene jungere Gruppe, und so schliesslich bestimmte 

 Korpertheile auf bestimmte Furchungszellen zuruckfiihren 

 lassen "] (13, p. 479). For the blastomeres are differentiated, in 

 size at least, from the first moment of their formation. The 

 development is here a visible mosaic-work, not one ideally 

 conceived by a mental projection of the adult characteristics 

 back upon the cleavage stages. The principle of "organ- 

 bildende Keimbezirke " has here a real meaning and value, and 

 this would remain true even if it should hereafter be shown 

 that both of the first two blastomeres of Nereis, if isolated, 

 could produce a perfect embryo. How then shall we reconcile 

 this case with that of AmpJdoxus ? The answer to this question 

 becomes comparatively simple if we regard the ontogeny as a 

 connected series of interactions between the blastomeres in 

 which each step conditions that which succeeds. The character 

 of the whole series depends upon the first step, and this in turn 

 upon the constitution of the original ovum. In Nereis the 

 mosaic-like character appears from the beginning decause of the 

 inequality of the first cleavage, and this conditions the entire 

 subsequent development through the peculiar inter-relations 

 established by it. The cause of the inequality must lie in the 

 undivided ovum, and it seems to me must in the last analysis 

 be sought in the constitution of the original germ-plasm. It 

 might be objected that the inequality may depend simply on an 

 unequal horizontal distribution of yolk, and not upon the 

 idioplasm. As a matter of fact no such inequality of distri- 

 bution is visible in the actual ovum (an extraordinarily 

 favorable object for examination); but waiving this rejoinder, 

 the objection only shifts the question further back. If such 

 inequalities exist they must be determined by a definite cause, 

 since the size-ratio of the first two blastomeres does not 

 perceptibly vary and we are in the end thrown back upon the 

 germ-plasm in every attempt to find this cause. Very numerous 

 facts support Roux's conclusion that the form of division in 

 cells is controlled by internal as well as external factors ; and, 



