20 BIOLOGICAL LECTURES. 



mere arises at a definite time, in a definite way, divides into a 

 definite number of cells, each having definite characters, and in 

 the end gives rise to a definite part. In such cases, as Wilson^ 

 has well said: " The development is a visible mosaic work, not 

 one ideally conceived by a mental projection of the adult char- 

 acteristics back upon the cleavage stages." Especially in the 

 case of the annelids and mollusks the cleavage is a mosaic 

 work more perfect than anything described by Roux, almost 

 every organ of the larva being represented by a differentiated 

 cell or group of cells before gastrulation is completed. 



On the other hand, no such definiteness is known to exist in 

 most cnidaria, echinoderms, and vertebrates, and is, in fact, 

 denied by several excellent observers. In such cases the cleav- 

 age is equally inconstant, indefinite, and devoid of morphological 

 sisnificance, whether one conceives with Whitman that the 

 unsegmented Q.gg is mapped out into " germ regions," which 

 are traversed in various directions by the cleavage planes, or 

 whether one holds with Driesch that no such "preorganization" 

 of the Qg^ exists, and that " by cleavage perfectly homogeneous 

 parts are formed capable of any fate." 



Obviously the same considerations apply to the axial relations 

 of the cleavage planes and, in case one denies the principle of 

 His, to the polarity of the unsegmented ^gg. In all cases in 

 which the cleavage has a mosaic character the relation of the 

 egg-axis and of the planes of cleavage to the embryo or adult 

 are perfectly definite and constant, and in many cases in which 

 the cell lineage has not been followed and in which the mosaic 

 character of the cleavage has not been directly recognized the 

 constant relation of the planes of the first and second cleavages 

 to the future planes of symmetry would indicate that the blasto- 

 meres bear constant relations to future organs. Whereas in 

 those cases in which the egg-axis or the position of the early 

 cleavage planes is inconstant the individual blastomeres can 

 bear no constant relation to adult structures. 



Confusion has already arisen through a failure to distinguish 

 these two types of cleavage; much of the recent experimental 



1 Wilson, E. B., " The Mosaic Theory of Development," Biological Lectures, 

 Wood's Holl, 1893. 



