204 



BIOLOGICAL LECTURES. 



conclusion follows that the form of the body at any period of 

 development is a resultant of {a) the relative size of the cells 

 which constitute the several parts, {b) the direction of the cleav- 

 age to which the cells owe their position, and (r) the rate of 

 cleavage of the various blastomeres, i.e., the intervals of time 

 between successive divisions, by which the number of cells in 

 any part is determined. If we could discover the factors 

 which determine respectively these three component features 

 of development, we should have material for an explanation of 

 the origin of the specific form of the animal body, embryonic 

 or adult. 



The problems relating to the rate of cleavage and the size 

 of the resulting cells have received much attention from vari- 

 ous investigators, and we will limit ourselves for the present to 

 a consideration of the third factor — the rate of cleavage. The 

 effect of the rate of cell-division in determining the form of the 

 embryo is well shown in the early stages, especially in those 

 forms in which the cleavage is "constant and determinate." 

 When the blastomeres in one part divide faster than those in 

 another, important changes take place in the form of the em- 

 bryo. The "teloblasts" in Clepsine, Rhynchelmis, and Lum- 

 bricus, for example, early give rise by their rapid divisions to 

 the germ-bands of the nerve-cords, nephridia, muscles, and 

 other portions of the ventral plate of the trunk, while the 

 divisions of the cells near the apical pole are less rapid, and 

 this region changes more slowly. If the form of the embryo is 

 affected by the differences in the rate of cell-division in 

 certain regions, our problem is to find out the factors which 

 account for these differences. In seeking them, let us first 

 examine the cell-division in the early stages of the embryo, and 

 afterwards those which occur later in the life-cycle. 



The "cleavage stages" in certain annelids and mollusks are 

 especially favorable for our purpose, because the behavior of par- 

 ticular cells can be followed with precision, the cells being few 

 and their manner of division constant. In the annelid Amphi- 

 trite, up to about the 64-cell stage, the differences in the rate 

 of division among the various blastomeres are comparatively 

 slight, so that all sixty-four cells belong to the same generation 



