The Spermato genesis of Aplopus mayeri. 25 



into this cytoplasmic fin. It is attached to the nuclear wall by a distinct 

 chromatic granule, probably a centrosome. Up to this point no structure 

 could be definitely decided upon as a centrosome in any of the mitoses, 

 and it is only rarely that even an indication of an aster can be observed. 

 However, the spindle fibers are always distinct and come to a definite 

 point at the poles. Were the centrosome not here pointed out by the 

 attached axial filament it would very probably escape notice among" the 

 various minute chromatic granules of the peripheral zone of the nucleus. 

 The nucleus at this stage frequently shows a polar cap of a material that 

 stains intensely in iron hematoxylin. This structure is not stained with 

 methyl green or thionin and probably represents the head-cap or acrosome 

 of the adult spermatozoon (figs. 136 and 137). 



The axial filament now enlarges proximally (fig. 127). While this 

 structure elongates distally and sends a slender thread into the long cyto- 

 plasmic tail, it differentiates into a proximal stout neck definitely marked 

 off from the distal filament, and represents the future middle-piece of the 

 spermatozoon. Successive stages with and without the accessory chromo- 

 some are shown in figures 128, 129, 130, and 131. The mass of cyto- 

 plasm surroundng the axial fiber subsequently becomes the cytoplasmic 

 fin of the tail, spirally arranged about the filament (fig. 136). 



Thus far in the metamorphosis the nucleus has remained approximately 

 spherical and is surrounded by a thin cytoplasmic envelope. Presently the 

 cytoplasm disappears, the nucleus becomes oval in shape, and the proximal 

 end of the middle piece widens and flares so as to form a concavity to 

 receive the nuclear convexity. The head-cap is conspicuous and the acces- 

 sory chromosome retains its compact spherical shape. Later stages show 

 processes of disintegration by fragmentation and karyolysis (figs. 133 and 

 134) and its final disappearance (fig. 135). The spermatozoon undergoes 

 still further changes of form, until in its final stage the nucleus is com- 

 paratively small and the middle-piece large. The latter has typically a 

 cigar-shape and the nucleus is approximately spherical, with a depressed 

 cone-shaped head-cap (fig. 136). 



The mature spermatozoon presents a very strange phenomenon in re- 

 gard to its staining reaction to recognized selective chromatin stains. 

 Figure 136 shows a spermatozoon stained in iron hematoxylin; figure 137 

 shows a spermatozoon of similar age stained with methyl green (or thionin). 

 It will be observed that the chromatic portions are exactly reversed as in- 

 terpreted by the two stains. The portion picked out by the methyl green 

 corresponds with the nucleus (head) of other spermatozoa, and since this 

 is a very selective chromatin stain, it probably definitely marks the true 

 limits of the nucleus. The reaction obtained with the iron-hematoxylin 

 stain, however, yields a very happy result in that it permits the observation 

 of the disintegrating accessory chromosome. Figures 138 and 139 show 



