230 bulletin: museum of comparative zoology. 



better preservation than that fixed by any other method. The earliest 

 stage of the chromosomes derived from the germinative vesicle that has 

 come under my observation is shown in Plate 1, Figure 2. The outline 

 of the germinative vesicle has only partially disappeared, and the chro- 

 mosomes are still indefinite in number and unlike in form. Their ar- 

 rangement on the spindle can hardly be said to have more than begun. 

 I have no stages between this condition and that represented by Figure 

 4, but I have in other specimens abundant corroboration of the appear- 

 ance which Figure 4 shows in regard to the method of separation of the 

 chromosomes in the first maturation division (compare Figure 1). In 

 the first maturation spindle I have always found sixteen chromosomes, 

 some of them more or less completely divided, others not only divided 

 but separated a short distance. Examination of Figure 4 will show that 

 some chromosomes are considerably longer than others. The shorter 

 ones lie end to end on the same spindle fibre. These I take to be 

 chromosomes which have completed their separation, and have now 

 begun to move toward their respective poles. At the middle point of 

 the longer chromosomes, which corresponds with the equator of the 

 spindle, there is an " elbow." Chromosomes in which this elbow is less 

 prominent, are longer than those in which it is large. The meaning is 

 quite clear. The appearances seen in the chromosomes of the first 

 maturation division of Limnsea are due to the more or less complete 

 splitting and separation of elongated chromosomes. 



Leaving now for a time the discussion of the phases immediately fol- 

 lowing metakinesis in the first maturation spindle, I shall take up the 

 consideration of the chromosomes in the prophase of the second matura- 

 tion division. Figure 11 (Plate 2) illustrates a condition which repre- 

 sents partly the prophase and partly the metaphase of division : that is to 

 sav, some of the chromosomes are undivided and some have just divided. 

 The chromosomes of this spindle are distinctly dumb-bell shaped, and 

 lie with their long axes parallel with the axis of the spindle. Here, as 

 in the first maturation spindle, I find sixteen chromosomes, all of which 

 are arranged on the outer, thicker fibres of the spindle, never, as is 

 usual in Limax, through the axis of the spindle. Careful examination 

 of the successive sections from which Figure 1 1 was constructed showed 

 that some of the dumb-bell shaped chromosomes were completely divided 

 across the "handle," and that migration from the equator had barely 

 begun. ISTot only is the form of the chromosomes in the prophase of 

 the second maturation division that of a dumb-bell, but even as early as 

 the telophase of the first maturation division all the chromosomes, both 



