48 THE EARLY DEVELOPMENT OF MAMMALS. 



ing protoplasm, causing the nucleus to become more or less shriveled. The 

 second step is the dissolution of the membrane of the nucleus so that the nuclear 

 contents are brought into direct contact with and partly mixed with the proto- 

 plasm of the cells. The third step, which in time more or less accompanies the 

 second, is the gathering of the chromatin of the nucleus into a definite number of 

 separate granules or chromosomes. These chromosomes are always conspicuous 

 and are larger than those formed during ordinary cell division. Their number is 

 also highly characteristic. As is now well known, there appear during the process 

 of indirect cell division a fairly definite number of chromosomes, a number which 

 is characteristic for each species. In numerous cases it has been observed that 

 the number of chromosomes in the maturing egg-cell is exactly one-half of that 

 found during the ordinary cell divisions of the same species. The chromatin 

 granules lie at first irregularly. Fourth, 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 possible, 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 radial. 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 undis- 

 covered 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 chro- 

 matin have each divided into two parts, and one-half of each fragment 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 tq be two plates 

 within the spindle. The translation of the groups of chromatin continues until 

 they reach the end of the spindle. The achromatic threads then break through 

 in the middle, and thus the 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 



