220 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the variation in the centrosphere as to render it advisable, and even 

 necessary, to discuss the two structures together. I shall first take up 

 the results obtained from the study of the maturation of Limax maximus, 

 and then follow with a more complete account of the history of the cen- 

 trosome and centrosphere in Limnsea elodes. 



1. Limax. 



(a) First Maturation Spindle. 



The earliest stage of the egg of Limax maximus that I have found is 

 one showing the first maturation spindle fully formed (not figured ; 

 compare with next older stage, Plate 3, Figure 16). The spindle, with 

 all the chromosomes in the telophase, has moved half the length of the 

 egg radius toward the animal pole. At either end of the spindle is to 

 be seen a large distinct centrosphere, composed of a central, pale, reticu- 

 lated area, and a very thick wall. The wall is nearly one third as thick 

 as the diameter of the whole sphere. Figure 1 6 represents a later stage, 

 but does not differ essentially from the earlier one as far as the condition 

 of the centrosphere is concerned. Careful examination of the centro- 

 sphere in both shows that the very fine reticulum of the clear region at 

 the centre is, to all appearances, continuous with the more compact re- 

 ticulum composing the wall. The astral rays emerge from the outer 

 portion of this wall, but, because of the density of the substance com- 

 posing the wall, it is impossible to say whether or not the central retic- 

 ulum is continuous through the wall of the centrosphere with the astral 

 fibres. The thickened wall of the centrosphere and the fine central 

 reticulum are not occasional phenomena, but are present in every first 

 maturation spindle in Limax maximus up to the stage represented in 

 Plate 3, Figure 16. The most thorough search through specimens fixed hy 

 different methods and stained for varying lengths of time has failed to re- 

 veal any trace of a centrosome in the kind of centrosphere just described. 

 Between the stage represented in Plate 3, Figure 16, and the one repre- 

 sented in Plate 3, Figures 17 and 21, is a gap in the condition of the 

 centrosome which I am not able to bridge over by intermediate stages. 

 However, the stages are not so far apart that the changes through which 

 the centrosphere would pass may not be fairly inferred. 



Whatever the influence that causes the condition of the centrosphere, 

 it is clear that the' process is one of increase in volume of the central 

 finely reticulated area. Figures 17 and 21 are particularly instructive 

 in this regard. In each two centrosomes have made their appearance. 



