502 PROCEEDINGS OF THE AMERICAN ACADEMY. 



somes of the first spermatocytes have heen studied in three species : 

 in Scolopendra heros (Blackman, :01, :03, :05), in Scolopendra sub- 

 spinipes (Blackman, :05*), and in Scutigera forceps (Medes, :05). In 

 these species the essential phenomena are similar, although the details 

 of the processes vary considerably. In all three forms the chromosomes 

 arise from chromatin stored in the karyosphere. The chromosomes 

 appear first as segments equal in number to the elements later taking 

 part in mitosis. These granular segments undergo first a longitudinal 

 and later a transverse cleavage, and soon take on the well-known cruci- 

 form shape characteristic of the heterotypical chromosomes of inverte- 

 brates. During the late prophase the chromatin granules fuse and the 

 tetrad becomes a homogeneous four-lobed chromosome. 



The formation of the tetrads in Scutigera varies in one interesting 

 particular from the typical process as observed in Scolopendra, as has 

 been shown by Medes (:05, p. 163): "Immediately after leaving the 

 karyosphere the chromosomes shorten into dense granular cords. These 

 now undergo a longitudinal cleavage and the double thread of chroma- 

 tin breaks up into a number of short portions of irregular size." These 

 portions are spherical in shape and of homogeneous consistency ; they 

 appear to be lobules of chromatin in a liquid condition. 



In describing the origin and behavior of the chromosomes of the first 

 spermatocyte in the forms used in the present paper, the observations 

 upon Lithobius sp. 1 will be given first, as here the phenomena are more 

 like those observed in other animals. Indeed, as regards the changes 

 in the chromosomes themselves, the nuclear processes are nearly identi- 

 cal with those in Scolopendra. The first change in the nucleus during 

 the prophase has to do with the appearance of the chromosomes, which, 

 as we have already seen, unite with the nucleoli during the early part 

 of the growth period to form several karyospheres. During the growth 

 period and vesicle stage this chromatin may be seen in the karyospheres 

 as dense masses or threads embedded in the nucleolar material. The 

 first change in the nucleus is in this chromatin. The dense masses 

 become more granular and more diffuse and move to the surface, thus 

 coming to lie outside the nucleolar portion. Strands of the chromatin 

 leave the karyosphere and lie free in the nucleus. This process con- 

 tinues until all of the chromatin has left the karyospheres. When this 

 is accomplished, the number of chromatic segments in the nucleus is 

 the same as the number of chromosomes participating in the succeed- 

 ing division. Figure 18 (Plate l) represents an advanced stage in 

 the disintegration of the karyospheres. Many of the chromosomes are 

 already free, while those which still remain in the karyosphere are 

 spread out as diffuse flocculent deposits upon the surface of the 

 nucleolar portions. 



