644 ZOOLOGY. 
of events: 1. Transportation of the germinal vesicle to the 
surface of the egg; 2. An absorption of the membrane of 
the nucleus or germinative vesicle and a change in the ger- 
minative spot ; 3. The portion of the nucleus surviving as- 
sumes a spindle-shape, this portion being largely formed 
from the nucleolus ; 4. One end of the spindle enters into a. 
protoplasmic prominence at the surface of the egg ; 5. The 
spindle divides into two halves, one remaining in the egg, 
the other in the prominence, the latter separating from the. 
egg and forming the polar cell ; 6. A second polar cell forms. 
in the same manner as the first, part of the spindle still re- 
maining in the egg; 7%. The part of the spindle remaining 
in the egg, after the formation of the second polar cell, is: 
converted into a nucleus, the female pronucleus, and finally, 
just before fertilization, the female pronucleus takes its po- 
sition at the centre of the egg. 
0 a > 
Fig. 589.—Development of the sperm-cells of a blind worm (Epicrium glutinosum). 
a, testis-cell; 5, the same, more numerous; ¢, d, ¢, becoming more numerous and’ 
finally forming spermatozoa (/f). Highly magnified. —After Minot. 
After this, the first step in the development of many-celled 
animals is the fusion of the protoplasm of the female pronu- 
cleus with that of the sperm-cell ; for this end the latter is 
exceedingly minute and provided with a vibratile cilium or 
‘‘tail,”’? so that it may force its way in toward the centre 
of the egg. These sperm-cells are developed in the testis. 
of the male. On close examination with very high powers of 
the microscope, certain cells, called ‘‘ mother cells,’’? may be 
found developed in fine tubules forming the gland ; these are 
known to possess several nuclei, which are destined to be- 
come spermatozoa (Fig. 539, a and 6); these multiply until 
they become very numerous, elongated, and packed side by 
