1917] Yocom: Some Phases of Sitcniiafogciirsis in the Mouse llTo 



Polar views of spindles of the division of the primary spermato- 

 cytes show twenty chromosomes. This number corresponds to that 

 found by Long and Mark (1911) in the eggs, as is to be expected 

 since in each case it is the haploid number. As further evidence that 

 twenty is the number, figure 2 (pi. 30,) shows an anaphase of the first 

 division with twenty cliromosomes migrating to each pole. The 

 scarcity of such cases indicates that the period of migration is rapid 

 compared with the other stages. Good polar views of the spindles of 

 more than fifty primary spermatocytes were examined, and the number 

 of chromosomes was found not to vary from twenty. 



SECONDARY SPERMATOCYTE 



There is no period of rest between the first and second divisions, 

 but the chromosomes become arranged on the spindle in an equatorial 

 plate for the second division. What the nature of the division is can- 

 not be stated at present, for it has not been possible to determine the 

 exact method of the formation of the chromosomes from the spireme, 

 but there is some indication that they form so that the first division, 

 which is transverse, results in the separation of the homologous 

 elements. If this is true, the second division follows a splitting of 

 the autosomes. There is one chromosome that does not divide in the 

 second division, and that will be described later. 



When viewed from the pole, the chromosomes of the secondary 

 spermatocyte appear so similar to those of the first spindle that it is 

 sometimes difficult to distinguish between them. There are two 

 characteristics, however, which aid in their recognition. First, the 

 secondary spermatocytes lie in a region of the tubule in which there 

 is a great number of spermatids; and, secondly, there are few late 

 spiremes to be seen in the region of the numerous secondary spermato- 

 cytes. After a little study the different kinds of cells can be distin- 

 guished beyond doubt. The points brought out in this paper are based 

 only upon the study of the cells about whieli there is no doubt as to 

 the stage of development. 



In polar views of spermatocytes of the second order it was noticed 

 that some had nineteen chromosomes, and others had nineteen in the 

 equatorial plate and one at a higher or lower level. Figure 4 (pi. 30) 

 represents a cell of the former type, and figure 6 .shows one of the 

 latter. An examination of figure 7 will indicate how such a condition 

 may be brought about. If the advancing chromosome has migrated 



