24 Papers from the Marine Biological Laboratory at Tortugas. 



the reticular stage and assumes its usual position close to the nuclear wall 

 (figs. 103 and 104). Occasionally it is double (fig. 105). As the ordinary 

 chromosomes take on the compact form and intense staining capacity, the 

 accessory becomes unrecognizable among them (figs. 107, 108, and 109) 

 in the equatorial plates. The latter give a chromosome count of 18 and 17, 

 representing daughter-plates from primary spermatocytes, with and with- 

 out the accessory chromosome, respectively. Plates with 18 chromosomes 

 show one large U-shaped body (fig. no). This, however, while usually 

 peripheral, is never greatly eccentric, neither here nor in the equatorial 

 plate of the first mitosis, as frequently represented in other insect forms. 

 Plates giving a count of 17 lack the U-shaped chromosome (figs, in and 

 112). The chromosomes at metaphase have the same characteristic dumb- 

 bell shape as they had in anaphase of the previous division. While most 

 of the chromosomes are already separating in metakinesis, a pair is just 

 entering the spindle with their long axes perpendicular to the fibers (fig. 

 115). This pair occasionally has its distal ends apparently fused at this 

 stage (fig. 113). It represents the fission products of the accessory chro- 

 mosome which has undergone an equation division similar to that of the 

 ordinary chromosomes of this mitosis. This pair is seen to lag behind in 

 the anaphase and even in the late telophase (figs. 116, 117, and 118), 

 Obviously only two out of every four spermatids can have the accessory 

 chromosome, and actual count of spermatids in several cysts corroborates 

 this theoretical conclusion. Figure 114 shows four contiguous secondary 

 spermatocytes in the equatorial-plate stage probably daughter-cells of two 

 adjacent primary spermatocytes giving a chromosome count of 17 and 18 

 alternately. The two cells with 18 chromosomes show an odd large U-shaped 

 element, the accessory chromosome. In the final stages of the telophase the 

 ordinary chromosomes again pass into the nuclear reticulum, but the acces- 

 sory chromosome remains intact as a more or less dumb-bell-shaped body 

 (fig. 120). 



SPERMATID AND SPERMATOZOON. 



Figure 121 shows three spermatids, two of which contain the accessory 

 chromosome. The latter may assume various shapes in the spermatid 

 (figs. 122, 123, and 124) and is usually again closely applied to the nuclear 

 wall. In the younger spermatids (fig. 121) the chromatin, which is very 

 sparse in amount, is arranged in clumps, mostly close to the nuclear wall. 

 During the later stages of metamorphosis into a spermatozoon the nuclear 

 contents assume a chromatic reticular character. Small karyosomes are 

 abundantly present, and the accessory chromosome now assumes a spheroidal 

 shape and more or less central position (fig. 125). Occasionally dark-stain- 

 ing granules of eliminated basichromatin are seen in the cytoplasm (fig. 126). 



The spermatid begins to lengthen its cytoplasmic body into a blunt tail 

 (fig. 125). Presently a very delicate axial filament begins to grow out 



