THE SPERMATOGENESIS OF DOMESTIC MAMMALS. 363 
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Text-fig. 1. Chromosomes of metaphase plate of first reduction division. 
a, nineteen chromosomes (four large-, eleven middle- and four small-sized ones) ; 
b, eighteen chromosomes (four large-, ten middle- and four small-sized ones), 
, The reduction :—The eighteen bivalent chromo- 
somes become arranged in the equatorial plate of the 
| first maturation spindle with their short axis parallel to 
that of the spindle (Figs. 37, 40, Text-fig 2.) The 
Ey Gide view of point of conjugation of two univalent chromatin 
chromosomes in the first spiremes thus lies in the equator of the spindle, where 
En Ei the division takes place (Figs. 37, 40, 41, 43). No 
longitudinal split of the chromosomes, therefore, can be recognised, and no 
typical tetrads are formed in this stage (Figs. 31, 37, 43). 
The accessory chromosome :—In the metaphase of this division (the first 
reduction division) the oval or the heart-shaped odd chromosome moves undi- 
vided toward one pole of the spindle in advance of the ordinary chromosomes 
(Figs. 38, 39, 43). Sometimes it can be distinguished from the other chromo- 
somes even at the anaphase (Fig. 44), but it usually becomes indistinguishable 
later, when the chromosomes collect so closely together that their individual 
outlines are entirely lost to view. It is, however, certain that one half of the 
daughter cells thus formed contains the accessory chromosome, while the other 
half does not. 
IV. The Secondary Spermatocyte. 
The resting stage :—As the resting stage of the secondary spermatocyte is 
rarely found in the horse, the duration of this stage appears to be very brief. 
A similar condition is recorded by WopsEDALEK (713) in the pig, by JoRDAN 
(11) in the opossum, and by Srevens (11) in the guinea-pig, but curiously 
enough WopsEDALEK (’14) denies the occurrence of this stage in the horse. 
Approximately one half of the cells of the secondary spermatocytes in the 
resting stage contain one large and several small chromatin masses (Figs. 47, 
