64 1? - !" I-l'TMAN. 



central spindle and of the aster. Whatever the origin and nature 

 of the centrosomes may be at this time it is at least something 

 that will take a stain and that has a definite location. After 

 the second division, at which time it lies at the ends of the 

 spindle again, it seems to disappear until a dark staining granule 

 appears at one side of the nucleus from which the axial filament 

 seems to be growing out. I have not traced the centrosome 

 around to discover whether the two are identical or not but from 

 the results on other animals it undoubtedly is. From this stage 

 on then, it would form the middle piece of the sperm. 



THE CHROMATIN NUCLEOLUS. 



McClung (4) has described in the germ-cells of certain grass- 

 hoppers a body which he calls the accessory chromosome. Pre- 

 vious to this discovery of this body the "chromatin nucleolus" 

 had been described by Montgomery (10) in the Hemiptera. 

 More recently the discussion of heterotypic chromosomes has 

 been given special importance by the papers of Stevens (13) and 

 Wilson (20, 21) especially in connection with the theory of sex- 

 determinants. 



The divisions in the nuclei of Platyphylax show a body which, 

 while it seems to have something in common with these described 

 structures, is in other respects quite different. Its behavior has 

 been reserved for this separate discussion. 



The various changes undergone by this body have been fol- 

 lowed to some extent both in the spermagonial and reduction 

 divisions. As the cells, and consequently this chromatin nucleo- 

 lus, are larger in the reductions divisions it will be described there 

 first. The nucleus of the young spermatocytes contains a nu- 

 cleolus that stains typically both in the triple and the iron 

 haematoxylin. This body is either spherical or ovoid in shape. 

 In the preparatory stages of division it begins to lengthen and 

 become spindle shaped. It frequently lies twisted over on it -dl 

 or is spoon-shaped at this time (Fig. 15). 



In a later stage when the chromatin has gone into the s\ naptic 

 condition this body seems as a rule to be somewhat smaller in 

 diameter as though it were spun out as the other chromatin has 

 been (Fig. 16). It does not lie among the chromatin strand-, but 



