THE DEVELOPMENT OF 



365 



forms the germinative area of the egg, a little protoplas- 

 mic cup, which rests upon one end of the large yolk. In a 

 few hours the process of segmentation divides this cup up 

 into a layer of cells, the blastoderm (Fig. 191, c). It is 

 rather difficult to find the early stages of segmentation, 

 but the process is as follows : a groove or furrow makes 

 its appearance on the surface of the germinative area, and 

 divides it into halves. A second furrow, at right angles 

 to the first, then divides it into quarters, and two more 

 into eighths, and so on, until it is divided up into a 

 number of wedge-shaped cells, with their pointed ends 

 meeting at the centre, and their broad ends at the periph- 

 ery of the germinative area. The tips of the central 

 ends are then segmented off, as a central ring of small 

 cells, another ring is formed outside the first, and so on, 

 and the cells which are thus formed are also divided up 

 into smaller ones. In this way the germinative area is 

 cut up into a layer of blastoderm cells, 

 as shown in Fig. 191, with the bases 

 of the wedge-shaped cells, or segmen- 

 tation pyramids around its edge. 



FIG. 191. An egg in which the process of 

 segmentation is somewhat advanced. (Drawn 

 from nature by W. K. Brooks. ) 



a. Egg-shell. 6. Space between the shell 

 and the yolk, filled with transparent albumen, 

 c. Cap of segmentation spherules, m. Micro- 

 pyle. 



FIG. 191. 



The cells of the blastoderm are not very well marked 

 in a living egg, but when treated with borate of carmine, 

 to which a very small quantity of one-tenth per cent solu- 

 tion of osmic acid has been added, they become very con- 

 spicuous. Fig. 192 represents the edge of the blastoderm 



