440 HOLMES. [Vol. XVI. 



cleavage plane does not coincide with either of these axes, 

 bilateral cleavage, when it occurs, is of the indeterminate type. 

 The converse proposition also appears to hold good, viz.^ where 

 the cleavage is determinate and the first cleavage plane coincides 

 with the median or transverse axis, the cleavage is of the bilat- 

 eral type. If these rules have exceptions, they are sufficiently 

 general not to be devoid of significance. The cleavage of the 

 ovum is influenced by a large number of factors, both internal 

 and external. Of these factors the degree of organization of 

 the egg and the direction of the first cleavage plane with refer- 

 ence to the planes of symmetry of this organization play, I 

 believe, an important part. 



Reversal of Cleavage and Cell Homologies. 



Detailed study of the early developmental stages of annelids 

 and mollusks has brought to light numerous and striking points 

 of resemblance between the cleavage of various members of 

 these groups ; and the cleavage of the polyclades, as shown by 

 Lang's work in Discocelis, is so similar to that of the above 

 forms that it may properly be considered as belonging to the 

 same general type. Professor Wilson, in his paper on the 

 " Cell Lineage of Nereis," has called attention to the close 

 resemblances of the cleavage of the annelid Nereis to that of 

 the gasteropods Crepidula and Neritina and the polyclade Dis- 

 cocelis as follows : " Up to a late stage in the spiral period 

 (twenty-eight cells) every individual blastomere and every cell 

 division is represented by a corresponding blastomere and a 

 corresponding cell division in the embryo of the polyclade, and 

 in that of the gasteropod. In all three the first two cleavages 

 and the upper and lower cross furrows have the same relations. 

 In all, three groups of four micromeres each are successively 

 separated from the macromeres, — the first group in a right- 

 handed spiral, the second in a left-handed spiral, and the third 

 in a right-handed spiral, like the first. The micromeres of the 

 second and third groups alternate with one another so as to 

 form an outer belt of eight cells that surrounds the four primary 

 micromeres." 



