No. 2.] FORMATION OF THE FISH EMBRYO. 43 1 



the embryonic cells are too small and too irregular to make 

 it worth while to draw them. 



The main conclusion that must be drawn from these figures 

 is, that as the extra-embryonic area increases in extent, there 

 is also an increase in the area of each cell of which it is formed. 

 A corresponding flattening of these cells takes place at the same 

 time, so that when the yolk is finally overgrown an exceedingly 

 thin membrane is left to cover the extra-embryonic region, 

 germ-ring, and embryo. 



The main question to be decided is this : Will the enlarge- 

 ment of the individual cells be sufficient when all taken 

 together to account for the greater enlargement at each stage 

 of the extra-embryonic region .'' We must also determine 

 whether the number of cells in this area is the same through- 

 out or becomes greater. In order to get an answer to these 

 questions with as small a chance of error as possible, I made 

 spheres of clay, as much larger than the real fish-egg as the 

 cells themselves had been magnified. In the spheres (mag- 

 nified 370 times) cells or groups of cells were traced and then 

 a count made of the result. Three stages were selected, 

 one corresponding to an embryo at 1 1 a.m. (Fig. 2, A), one at 

 3 P.M. (Fig. 4, A), and one at 7,05 p.m. (Fig. 6, A). In each case 

 the embryonic area and the germ-ring were also sketched on 

 the enlarged Qgg, and their areas excluded from the part covered 

 by the extra-embryonic cells. 



The embryos at 11 a.m. were found to have 1785 cells in 

 the extra-embryonic region. Both sphere and cells were mag- 

 nified 370 times, and the superficial area of the extra-embryonic 

 region was equal to 125 square centimeters. A square of 

 5 centimeters on each side (=25 sq. cm.) contained 357 cells. 



At 3 P.M. the number of cells was found to be 4494 in the 

 extra-embryonic region. There were about 1050 square cen- 

 timeters, each 25 containing 107 cells. 



At 7.05 P.M. there were 6040 cells in the extra-embryonic 

 region. There were 2000 square centimeters and each 25 con- 

 tained ']6 cells. 



These results show that, as the area of the extra-embryonic 

 portion increases, the size of each cell increases and the number 



