IQO CONK LIN. [Vol. XIII. 



less completely set aside. It seems quite certain, therefore, 

 that the cause of the bilateral form of cleavage is an intrinsic, 

 not an extrinsic, one. If it be true that there are cases of 

 bilateral cleavage which have no reference to the bilaterality 

 of the adult, as Miss Clapp's ('91) observations on the toadfish 

 and Morgan's ('93) on certain teleosts indicate, it can only be 

 explained, so far as I can see, by supposing that the same 

 causes which operate to produce bilaterality in the adult may 

 operate independently on the cleavage stages, producing bilat- 

 eral symmetry which has no connection with that of the adult. 

 This probability seems to me so remote that I think it more 

 likely, considering the extensive shiftings and rotations of blas- 

 tomeres which have been observed in some animals, that the 

 bilaterality of cleavage is only an early appearance of the final 

 bilaterality with which it is directly continuous, though perhaps 

 only after extensive shiftings of cell groups, or even of entire 

 layers, have occurred. The further possibility remains that in 

 some cases apparently bilateral cleavages are not really bilat- 

 eral, but are radial, as is the case with the first cleavage in 

 Crepidula. 



(d) Determinate and Indeterminate Cleavage. 



In only a comparatively small number of animals, so far, 

 has the history of individual blastomeres been traced through 

 the development to the organs which they ultimately form. 

 In a few cases, however, among such widely separated groups 

 as Annelida, Gasteropoda, Lamellibranchiata, Arthropoda, and 

 Tunicata, this has been done in the case of a few cells, and 

 with the constant result that, under norinal conditions, definite 

 cells in any given animal invariably give rise to definite struc- 

 tures in the embryo or the adult. Such cells are not only 

 identical in origin and destiny but also in shape, size, and 

 developmental history. Such definiteness in the origin, form, 

 and history of blastomeres leads irresistibly to the view that 

 the history 'of each cell in such ova is, under normal circum- 

 stances, predetermined and always in the same way and to the 

 same end. For all such kinds of cleavage I propose the name 

 determinate cleavage. 



