226 bulletin: museum of comparative zoology. 



maximus — has not yet begun in the specimen from which Figure 8 

 was drawn. In many other cases, however, at a slightly later stage the 

 centrosphere shows a decided increase in size, as is seen, for example, 

 in Plate 2, Figure 7. The peculiar arrangement of the chromosomes, 

 the disappearing spindle fibres, and the enlarged centrosphere shown in 

 Figure 7 represent a stage I have seen many times. In earlier stages 



I have never seen the centrosphere as large as it is in Figure 7. This 

 phenomenon of an increase in the size of the centrosphere which has 

 performed its function, was described by Miss Esther F. Byrnes for 

 Limax agrestis in a paper read at a meeting of the American Morpho- 

 logical Society in December, 1896. 



b. Second Maturation Spindle. 



I have searched with considerable care for evidence of two centro- 

 somes within the enlarged centrosphere of the first maturation spindle of 

 Limnsea elodes, but I have found them in only a single case (Plate 1, 

 Figure 4), a very early stage in the formation of the first maturation 

 spindle. There is no evidence of a spindle forming between the centro- 

 somes, as, indeed, nothing of the kind could be expected at so early a 

 stage. Hence I am unable to say, as far as Limnsea is concerned, what 

 relation the enlarging centrosphere bears to the formation of the second 

 maturation spindle ; whether the new spindle is formed de novo within 

 the still persisting centrosphere, or whether the centrosphere disappears 

 before the second maturation spindle comes into existence. 



A good example of a second maturation spindle near the height of its 

 development is shown in PlaJ:e 2, Figure 11. In this case the two astral 

 figures are conspicuously unlike. Frequently I have found in Jirst matu- 

 ration spindles one centrosome differing from the other in size, and also 

 the centrospheres differing in condition, but the variation shown in Figure 



II appears to be of quite another nature. In this case one centrosome, 

 the peripheral one, has no centrosphere surrounding it, and the centro- 

 sphere at the deep pole of the spindle is flattened in a plane per- 

 pendicular to the axis of the spindle. I have not enough material of 

 the proper stages to allow me to make a careful study of the second 

 maturation spindle with reference to the centrosomes and centrospheres, 

 and hence do not pretend to say whether Figure 11 represents a typical 

 condition. 



The phenomenon of concentric centrospheres is not confined to the 

 first maturation spindle. The aster of the second maturation spindle 

 shown in Plate 2, Figure 13, is not altogether like that shown in Figure 



