508 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The observations of P. Bouin (:00, :0l) upon the same species are 

 practically identical with the foregoing, and those of Bouin et Collin 

 (:0l) extend these observations to Geophilus linearis. These authors 

 (p. 106) reach the following conclusions regarding the modified spin- 

 dle: "De cette ^tude sur le fuseau chez le Geophilus linearis nous 

 nous croyons autorisds h conclure qu'il existe (^galement chez ce Myria- 

 poda une independence complete entre le fuseau d'une part, les spheres 

 et les corpuscles polaires, d'autre part, independence evidente au stade 

 de la m^tacin^se, plus difficilement observable, mais tr^s vraisemblable 

 cependant d6s prophases de la mitose." 



In Scutigera forceps (Medes, :05) much the same condition exists in 

 the fully formed spindle, although earlier stages in its formation pre- 

 sent some important differences, and at this time more closely resem- 

 ble corresponding stages in the small spermatoc3i:e of Scolopendra 

 (Blackman, :01, :05). The centrosomes at first lie at the poles of the 

 spindle. Later, however, they leave this position and migrate to the 

 cell membrane, retaining no apparent connection with the nuclear 

 spindle. During the metaphase the author (Medes, p. 168) states: 

 " The mutual independence of the spindle-fibres and the astral rays in 

 Scutigera is the more clearly shown by the great dissimilarity in their 

 structure and general appearance ; for while the latter are very fine 

 and delicate, . . . the former are heavy, stain an intense black, and 

 present the appearance of wires, the ends of which may be distinctly 

 seen lying free in the cytoplasm." 



In the present study of the achromatic structures the cells of Litho- 

 bius mordax have been used almost entirely, although observations 

 showing that the changes are common to the genus have been made 

 on the other two species. It will be seen that in general my obser- 

 vations on the achromatic structures are largely confirmatory of those 

 on the European species of Lithobius made by Bouin (:00, :Oi)and by 

 Meves und von Korff (:0l). 



In the prophase of Lithobius mordax the centrosomes were first 

 seen in cells in which the nucleus is about in the stage represented in 

 Plate 1, Figure 23. At this stage there are two, one lying at each 

 side of the nucleus and about midway between this structure and the 

 cell membrane. Each centrosome has already begun to divide to pro- 

 duce the centrosomes of the second spermatocyte mitosis, and is now 

 a small, darkly stained, dumb-bell shaped body. It lies in a small, 

 clear area of the cytoplasm, and is surrounded by astral radiations 

 which, though very fine and delicate, are still distinct from the cyto- 

 plasmic reticulum. 



In later stages the centrosomes move through the cytoplasm toward 



