170 PROCEEDINGS OF THE ACADEMY OF [Feb., 



half the miml)er of univalent chromosomes present in the spermatogo- 

 nia, namely, one bivalent heterochromosome and nine bivalent ordinary 

 chromosomes. But at this stage there seems not to be possible a posi- 

 tive recognition of which is the heterochromosome. In a number of 

 cases after nine of the chromosomes were arranged in the equator and 

 some of them were beginning to divide (fig. 33), one (y) had not yet 

 taken up that position but lay nearer one spindle pole than the other. 

 This was the case, e.g., with four cells in exactly the same stage lying 

 in the same section of one testicular follicle, and in all of these the iso- 

 lated chromosome was of the same size and form, straight, and appear- 

 ing to consist of two closely apposed arms. It may be that this chromo- 

 some is the heterochromosome with which it agrees in general form and 

 size, but this could not be definitely determined ; ultimately it takes a 

 position in the equator and divides with the others. In fig. 32 is an 

 element, y, closely corresponding in size with it and with the hetero- 

 chromosome during the prophases; but I cannot say positively that 

 y of fig. 32 is the heterochromosome, though the probability of it is 

 evident. All these chromosomes become so placed in the spindle that 

 mantle fibres from one spindle pole are attached to one univalent ele- 

 ment, and mantle fibres from the other spindle pole to the other uni- 

 valent component of each bivalent chromosome. The longitudinal 

 split can no longer be seen, but previously it lay in the axis of each 

 univalent chromosome. These definitive chromosomes may be dumb- 

 bell-shaped, or as frequently irregularly V-shaped, ring-shaped, or in 

 the form of twQ parallel rods (Plate IX, figs. 31-33; Plate X, fig. 34). 

 But whatever the form, they become arranged so in the spindle that 

 the point or points of junction of the univalent components of each 

 lies in the equatorial plane. The early formation of these chromosomes, 

 their arrangement in the spindle, then their division (Plate X, figs. 34- 

 36) show that the first maturation mitosis is a reduction division and 

 separates from each other the univalent chromosomes of each pair. 

 There is no evidence that this is an equation division taking place 

 along the line of the longitudinal split — no evidence at any period 

 that a chromosome had become elongated in a line at right angles to 

 its original long axis. Each arm of a bivalent chromosome is a whole 

 univalent chromosome and not a split half of one ; and the long axis 

 of each arm is in the same line as its long axis at earlier stages. I 

 have tested the morphological evidence of this process very honestly 

 and fairly, for at the commencement of my study I was quite prepared 

 to find the first maturation mitosis an equation division. But it is a 

 reduction division. The di\dsion of certain of the chromosomes may 



