180 PROCEEDINGS OF THE AMERICAN ACADEMY. 



VII. 



CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF 



THE MUSEUM OF COMPARATIVE ZOOLOGY, UNDER 



THE DIRECTION OF E. L. MARK, XXXIX. 



ON SOME LAWS OF CLEAVAGE IN LIMAX. 



A PRELIMINARY NOTICE. 



By C. A. Kofoid. 



Communicated by E. L. Mark, January 10, 1894. 



The following is a statement of the results obtained from the 

 study of cleavage in Limax, and of the literature of cell lineage 

 in other invertebrates. It is desirable to confirm my results by a 

 study of cleavage in other forms before the publication of my 

 final paper, and it has therefore seemed best not to defer the 

 presentation of the conclusions to which I have arrived. 



A few words in regard to the usage of terms will be necessary. 

 The egg is regarded as having the animal pole uppermost, and 

 the terms right and left, upper and lower, are used as resident in 

 the egg itself. Or, to express it in another way, a miniature 

 observer is imagined as placed in the principal (vertical) axis of 

 the egg, with his head at the animal pole, facing the part or parts 

 of the egg under discussion, and the terms right and left, upper 

 and lower, are used as determined by this observer. By " a gen- 

 eration of cells " is meant all those cells which are removed from 

 the ovum by the same number of cell divisions, regardless of the 

 time of appearance or position of such cells, i. e. the word is 

 used in its literal sense. This is not the usage of Fol (75) or 

 Blochmann ('81), who employ the term in its literal sense with 

 reference to the blastomeres through the four-cell stage, but 

 thereafter use it to designate successive sets of four micromeres, 

 naming them in the order of their appearance in time. 



As is well known, cells cleave in sets of fours throughout the 

 spiral period of cleavage. The cleavage of the individual cells 

 of the set may be synchronous or successive, and the cleavage of 



