THE SPEaM.\TOGENESIS OF DOMESIIC MAMilALS. 223 



tlicm carofully in accordance ^^■^tl^ ilicir Imn nucl size, it ^vill lio pluiuly 

 shown that thoy arc represented by pairs, with the exception of the sox- 

 oliromosomes (Fig. 13). In the telophase all the chromosomes become so 

 clost>ly aggregated tlxat their individual outlines are entirely lost to view. 



The second reduction division : — In the second reduction division the 

 chromosomes have a tendency to gather into a mass, which makes it diiHculfc 

 to count thorn with certainty. In fairly good stained preparations the number 

 of chromosomes is estimated to be twenty, but tlie sex-chromosomes can not he 

 clearly distinguished (Figs. 59-62). 



As in the case of the horse and cattle the second pairing of the chromo- 

 somes reported by Gdyer ('10), Jordan ('12) and Wodskdalek ('13, '14, '20) 

 could not Ijo seen. It has also been recently denied b}- Hance ('17) in the 

 pig and by Allen ('18) in the rat It is very likely that such an apparent 

 l^airing of the chromosomes is produced as the result of a poor fixation of 

 the material in which the chromosomes are mjisied together. 



In the equator of this division all the chromosomes become so placed 

 that the transverse constriction of each of them coincides with the equatorial 

 plate, and" it is along this plane that they are divided at the same time, no 

 special chromosomes with different behavior being seen among them (Figs. 

 62-Gi). This constriction of tlie chromosome represents the longitudinal 

 splitting which is comparable to that of the somatic chromosome. If it is 

 so, it will be conjectured tliat the real character of the second division is 

 simply an equation division, the sex-chromosomes being also divided into two 

 by the longitudinal splitting like the ordinary ones. As in the first division, 

 in the anaphise of this division the chromosomes are distinctly separated 

 and for a short time maintain their individualities (Fig. 65). In the telophase 

 the chromosomes also gather together so closely tluit their individualities 

 become entirely lost to view (Fig, 66). 



The mitochondria : — During the first reduction division the mitochondrial 

 giiUiules lie outside the spindle, lieing scattered throughout the cell body 

 (Fig. 42). Sometimes linear an-angement of them is to bo seen in this stage 

 (Fig. 42). When the cells divide into two, they remain undivided and seem 

 to be equally distributed in the cytoplasm of the daughter cells. In the first 

 division most of the granules seem to attain thnv greatest size, Ijut it is 



