Further Examination of the Cell-Theory 217 



namely, to structural features of the ovum which influence 

 the early stages of cell-division, and the shape, size, struc- 

 ture and so forth, of the "blastomeres," i.e., the product 

 of the early egg-cell divisions. These phenomena have been 

 considered more significant than the differences between 

 eggs of different species above referred to, and have been 

 investigated by embryologists rather than by systematists ; 

 and it is to a large extent on evidence from this source that 

 the fact that the '''^gg is not one being and the embryo 

 another and the adult a third, but the Qgg of a human being 

 is a human being in tlie one-celled stage of development" ^'^ 

 has gained theoretical interest. 



Several of the most thoughtful embryologists who have 

 investigated the earliest stages of numerous animals by the 

 best modern methods have expressed views more or less 

 like this, and so are in accord with zoologists who have 

 been led to the comparative investigation of eggs by prac- 

 tical considerations. 



A quotation from E. B. Wilson will serve well as a start- 

 ing place for our inquiry as to what promorphology is in 

 this restricted sense. "It is a remarkable fact," writes 

 Wilson, "that in a very large number of cases a precise 

 relation exists between the cleavage products and the adult 

 parts to which they give rise; and this relation may often 

 be traced back to the beginning of development, so that 

 from the first division onward we are able to predict the 

 exact future of every individual cell. In this regard the 

 cleavage of the ovum often goes forward with a wonderful 

 clocklike precision, giving the impression of a strictly or- 

 dered series in which every division plays a definite role 

 and has a fixed relation to all that precedes and follows it. 

 But more than this, the apparent predetennination of the 

 embryo may often be traced still further back to the regions 

 of the undivided and even unfertilized ovum." ^^ 



This preordination of the future animal in the egg before 



