THE MATURATION OF THE OVUM. 37 



The chromatin granules lie at first irregularly. Fourthly, there arises a characteristic 

 spindle figure such as accompanies mitosis. The chromatin forms an equatorial 

 plate, each granule being associated with one of the spindle threads. The shape 

 of the spindle varies, as does also the distribution of the granules of the equatorial 

 plate. In guinea-pigs the ends of the spindle are pointed and the threads are 

 straight, the outline of the spindle being like a diamond; in the bat the spindles 

 are barrel-shaped and the threads are curved. In many cases it is known, and it 

 may be found to be true of all cases, that the axis of the spindle is at right 

 angles to the radius of the ovum. The nuclear spindle now changes its position, 

 becomes first oblique, and then r.adial. One end of the spindle lies close to the 

 surface of the ovum. The first step is the division proper. The spindle, driven 

 by an undiscovered power, moves centrifugally until it is partly extruded from the 

 egg. The projecting end is enclosed in a distinct mass of protoplasm which 

 gradually increases and soon becomes constricted around its base. The fragments 

 of chromatin have each divided into two parts (dyads), and one half of each frag- 

 ment moves toward one end, and the other half toward the other end of the 

 spindle. The half fragments of each set move together, hence there seem to be 

 two plates within the spindle. The translation of the groups of chromatin con- 

 tinues until they reach the end of ^the spindle. The achromatic threads then 

 break through in the middle, and thus t\ie original nucleus, or at least a part of it, 

 has been divided. There are now two masses of nuclear substance, one in the 

 ovum, the other in the polar globule. The result of the whole process is the 

 formation of two cells extremely unequal in size, and each containing in its nuclear 

 elements half the number of chromosomes characteristic of the body-cells. The 

 number of chromosomes has, therefore, been reduced, hence the term reduction 

 division. It will be noted that the actual reduction in the number of chromosomes 

 took place when they were first formed in the maturing ovum, while the spindle or 

 mitotic figure was developing. 



The second polar globule is produced a short time after the first. When this 

 occurs, the nuclear remnants in the ovum do not reconstitute themselves into a 

 membranate nucleus, as occurs in ordinary cell division, but they change directly 

 into a second spindle, which lies, as did the first, within the protoplasm of the 

 animal pole. This second spindle occupies an oblique position, or may even be 

 parallel with the surface. But it also soon takes up a radial position and pro- 

 duces a second polar globule in similar manner to the first. The dyads all divide, 

 and the ovum receives the half number of chromosomes, each of which represents 

 the fourth part of a tetrad (double chromosome). The second globule is usually 

 smaller than the first. 



It may also happen that the first polar globule may itself divide, so that three 

 polar globules appear. 



The Formation of the Female Pro-nucleus. The nuclear material which remains 

 in the main .ovum after the separation of the polar globules -is known as the 



