448 



HISTOLOGY 



takable U-shaped accessory chromosome in this position. In Figure 411, 

 21, are seen the ordinary chromosomes in the spindle at metaphase. 



The first maturation di- 

 vision is undoubtedly trans- 

 verse, separating whole 

 chromosomes, and is there- 

 fore a true reduction divi- 

 sion. The second maturation 

 mitosis is the equation divi- 

 sion (this being sometimes 



FlG. 412. Aplopus Mayeri. 25 and 26, res ting stages consummated precociously 



of second spermatocyte with accessory chromosome; ^ t P l nn V, fl cp r>f trip first 

 27, a pair of sister spermatocytes of second order, 



one of which has, and the other consequently has mitosis), both for the Ordl- 



SV^ a jSSS } cllromosome - x I4 - (Drawn nary and accessory chromo- 

 somes. In the first reduc- 

 tion division the accessory usually assumes a position on the spindle in 

 advance and to one side of 

 the other chromosomes (Figs. 

 22-23). In the later telophase 

 it almost invariably splits into 

 two portions, the result of a 

 separation of the two arms 

 of the U at the bend (Fig. 24). 

 Obviously, since the accessory 

 chromosome passes undivided 

 to one of the poles of the first 

 maturation spindle, the result- 

 ing daughter cells (secondary 

 spermatocytes) are of two 

 classes, i.e. those with the 

 accessory and those lacking 

 it (Fig. 27). A brief resting 

 stage ensues, during which 

 the nuclear wall is recon- 

 structed (Fig. 25) and the 

 mass of chromosomes disen- 

 tangles and diffuses into a 

 pale reticulum. 



Meanwhile the accessory 

 chromosome has remained 

 separate from the ordinary 

 chromosomes and has retained its characteristic form and position 

 in the nucleus. Very soon the reticulum passes through the regular 



FlG. 413. A plopus Mayeri. 2 8, 2 9, 30, three stages 

 in the second reduction division of the spermato- 

 cytes of the second order to form spermatids; 28 

 and 29 show equation divisions of the accessory 

 chromosome; 31, equatorial plates of four second- 

 ary spermatocytes ; two possess the accessory chro- 

 mosome while the other two do not ; 32 , telophase 

 of second reduction division ; accessory chromosome 

 not distinguishable among other dark staining 

 chromosomes. X 1400. (Drawn by H. E. JORDAN.) 



