Further Examination of the Cell-Theory 217 



namely, to structural features of the ovum which influence 

 tin- curly stages of cell-division, and the shape, size, struc- 

 lure and so forth, of the "blastomercs," 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 systernatists ; 

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

 the fact that the "egg is not one being and the embryo 

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

 is a human being in the one-celled stage of development" ls 

 has gained theoretical interest. 



Several of the most thoughtful embryologists who have 

 investigated the earliest stages of numerous animals by the 

 he.st 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- 

 ical considerations. 



A quotation from K. 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 



lv ilson, "that in a very large number of cases a precise 

 ation exists between the cleavage products and the' adult 

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

 traced back to the beginning of development, so that 

 >m 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 

 locklike 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 predetenninat ion of the 

 embryo may often be traced still further back to the regions 

 of the undivided and even unfertilized ovum." 1 



This preordination of the future animal in the egg before 



