THE MOSAIC THEORY OF DEVELOPMENT \\ 



ments of truth in the mosaic hypothesis ; and I will attempt 

 to indicate this modification by a comparison between Amphi- 

 oxns and Nereis. In the case of AmpJiioxus we have the clear- 

 est evidence that differentiation is, in a measure, dependent 

 upon the relation of the cell to the whole of which it forms a 

 part. The first visible differentiation in this case is at the 

 third cleavage, which consists in an unequal division of each 

 of the four blastomeres, so as to give rise to four micromeres 

 and four macromeres, the former giving rise to ectoblast only, 

 while the latter give rise to entoblast 

 and mesoblast as well {Diagram I). 

 If, however, the blastomeres of the 

 4-celled stage be separated (shaken 

 apart) the course of events is entirely 

 changed ; for in this case each divides 

 equally, not unequally, and ultimately 

 gives rise to a complete quarter-sized Uiagrvm I 



dwarf, instead of one-quarter of a 



normal embryo, as it would have done under ordinary cir- 

 cumstances. The character of the fourth cleavage is here 

 directly or indirectly determined in each cell by the relation 

 of the cell to its fellows ; and if this is true of any one 

 stage of the ontogeny, a very strong presumption is created 

 that it is true of all — that, in the process of progressive 

 differentiation occurring in the course of every animal on- 

 togeny, the character of each step is determined by the 

 condition of the entire organism. The ontogeny is, in other 

 words, a connected series of interactions between the 

 various parts of the embryo, in which each step estab- 

 lishes new relations, through which the following step is 

 determined. The character of the series, as a whole, depends 

 upon the first step, and this in turn upon the constitution of the 

 original ovum. In AinpJiioxtis differentiation proceeds slowly, 

 the earlier blastomeres show no appreciable divergence, and the 

 first stages show no trace of a mosaic work. In Nereis, on the 

 other hand, a mosaic-like character appears from the begin- 

 ning, because of the inequality of the first cleavage, which 

 conditions the entire subsequent development through the 



