194 THE CELL 



of the dumb-bell, and quickly enters the last stage of division 

 (Fig. 90 A). A ring-like furrow corresponding to a plane, which 



FIG. 90. Egg of a Sea-urchin when division is just taking place (f rom O. Hertwig, Embryo- 

 logy, Fig. 29). A A circular furrow cuts into the yolk and divides it in a plane which is 

 perpendicular to the centre of the nuclear axis and to the long axis of the dumb-bell. B 

 Egg of a Sea-urchin after division has taken place. In each of the division products a 

 vesicular daughter nucleus has been formed. The radial arrangement of the protoplasm 

 is commencing to become indistinct. Both figures are drawn from the living object. 



might be carried through the dumb-bell at right angles to its 

 longitudinal axis, develops upon the surface of the egg. This 

 rapidly penetrates more and more deeply into the egg-substance, 

 quickly dividing it into two equal portions, each of which contains 

 half of the spindle with a group of daughter segments, that is to 

 say half of the dumb-bell, and a radial system of protoplasm. 



When the division in two is nearly completed, the two portions 

 of the egg are in contact at a small portion only of their surfaces, 

 at the middle of the handle of the dumb-bell. When, however, 

 cleavage is quite finished, the whole of their division surfaces come 

 closely into contact with one another, so that they flatten each 

 other into nearly hemispherical bodies (Fig. 90 B). 



Meanwhile the nucleus has become visible in the living object. 

 Somewhere near the place where the head and the handle of the 

 dumb-bell merge, that is to say, at some little distance from the 

 centrosome, a few small vacuoles make their appearance, being 

 caused by the saturation of the daughter nuclear segments with 

 nuclear sap. After a short time these fuse together to form a 

 globular vesicle, the daughter nucleus (Fig. 90 B). The radiated 

 arrangement of the protoplasm grows gradually less distinct, and 

 makes way, if the cell prepares to divide a second time, for a new 

 double radiation. 



