1879.] 195 [ Minot. 
which determines the number of cells existing at a given time after 
impregnation. | 
We turn now to the consideration of the weight of the cells, which 
varies in several definite ways. 
4. The weight of the cells diminishes at every division and again in- 
creases until the cells regain approximately their original size. ‘This 
proposition becomes evident upon the mere comparison of the tissues 
of adult and young individuals of the same species, for we find that 
the cells retain about the same dimensions, although the size of the 
cell diminishes by half at every division, because the whole amount 
of matter originally in one cell separates into two. ‘To counteract 
this repeatedly recurring diminution the daughter cells must grow up 
again to the parental size — in the case of unicellular animals, in the 
embryo, in cartilage, etc., it can be actually seen that this restoration 
of the size occurs. We must regard this periodical vacillation as a 
general phenomenon. 
5. The weight of the cells may alter in one direction during several 
generations. ‘That in the course of development the cells do essen- 
tially change their size is evident from the fact that the cells of each 
germ-layer of the embryo are all of about the same size, while in the 
adult and differentiated tissues, the size varies greatly. Studies upon 
the exact succession of these changes have been published upon man 
by Harting! and upon insects by the two Landois,? the only special 
investigations upon this subject with which I am acquainted. Prob- 
ably however the changes in one direction are counterbalanced by 
changes in the opposite direction in other cells, and in any case the 
average weight of the cells cannot alter to any great extent. 
There is, however, one very important exception. The ege is 
always greatly larger than the histological elements either of its 
parent cycle, or of its own cycle. During the process of segmenta- 
tion the diminution in the size of the cells continues for a consider- 
able number of generations, during which there is either no, or at 
most a very slight, change in the total weight of the embryo; hence 
also the diminution increases as the cells multiply, until the normal 
size of the elements is reached, when the enlargement of the single 
cells counterbalances the diminution of size at each division, as above 
1 Harting. Recherches micrometriquer dur le Develop. des Tissus et des Or- 
ganes du Corps humain. Utrecht. 1845. 
2 Landois. Zeitschr. f. wiss. Zool., XV, (1865), 307. 
