REPRODUCTION, 59 
In viewing a section of the. ovary taken from a mammal at 
the breeding-season, ova and Graafian follicles may be seen in 
all stages of development—those, as a rule, nearest the surface 
being the least matured. The Graafian follicle appears to pass 
inward, to undergo growth and development and again retire 
toward the exterior, where it bursts, freeing the ovum, which is 
conducted to the site of its future development by appropriate 
mechanism to be described hereafter. 
Changes in the Ovum itself—The series of transformations 
that take place in the ovum before and immediately after the 
access of the male element is, in the opinion of many biolo- 
gists, of the highest significance, as indicating the course evolu- 
tion has followed in the animal kingdom, as well as instructive 
in illustrating the behavior of nuclei generally. 
The germinal vesicle may acquire powers of slow movement 
(amceboid), and the germinal spot disappear: the former passes 
to one surface (pole) of the ovum; both these structures may 
undergo that peculiar form of rearrangement (karyokinesis) 
which may occur in the nuclei and nucleoli of other cells prior 
to division ; in other words, the ovum has features common to 
it and many other cells in that early stage which precedes the 
complicated transformations which constitute the future his- 
tory of the ovum. 
A portion of the changed nucleus (aster) with some of the 
protoplasm of the cell accumulates at one surface (pole), which 
Fig. 63.—Formation of polar cells in a star-fish (Asterias glacialis) (from Geddes, A—K after 
Fol, L after O. Hertwig). A, ripe ovum with eccentric germinal vesicle and spot ; B—D, 
gradual metamorphosis of germinal vesicle and spot, as seen in the living egg, into two 
asters ; F, formation of first polar cells and withdrawal of remaining part of nuclear 
spindle within the ovum ; G, surface view of living ovum in the first polar cell ; H, com- 
pletion of second polar cell ; I, a later stage, showing the remaining internal half of the 
spindle in the form of two clear vesicles ; K, ovum with two polar cells and radial strice 
round female pronucleus, as seen in the living egg (E, F, H, and I from picric acid prepa- 
rations) ; L, expulsion of the first polar cell. (Haddon.) 
is termed the upper pole because it is at this region that the epi- 
thelial cells will be ultimately developed, and is separated; this 
process is repeated. These bodies (polar cells, polar globules, 
