284 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the axis of the spindle. During this change, it increases in size and in 

 the intensity of its staining. The centroplasm arises in a manner 

 similar to that described for the first maturation figure, appearing first 

 as a thin layer closely enveloping the centriole. The outline of the 

 centroplasm becomes somewhat crenate, but soon assumes an even ap- 

 pearance. The centriole, meantime, gradually becomes reduced in size 

 and regular in form. The only difference between this centrosome and 

 that of the first mitotic figure is that it is smaller than the latter. A 

 sphere is evident, although not very distinct. The medullary and cor- 

 tical layers may be distinguished in Figures 33 to 35. 



The chromosomes are drawn into the equatorial plane in a manner 

 similar to that described by MacFarland ('97) for Diaulula and by 

 Lillie (:0l) for Unio. In Figure 32 they are vesicular ; this seems to be 

 their only approach to a so-called resting condition. In appearance 

 they are identical with the chromosomes in the longer or indirect 

 pi'ocess, to be described presently, but the vesicular state occurs here 

 much later than in the other process. This vacuolated condition 

 gradually disappears, and the chromosomes again become solid and 

 squarish or oblong. Their form is such that it is impossible to say 

 whether division is longitudinal or transverse ; they are seen in the act 

 of separating in Figures 33 and 36. These changes are in many ways 

 similar to those observed in Chsetopterus (Mead, '98), Thalassema 

 (Griffin, '99), and Cerebratulus (Coe, '99). From this point forward 

 the two processes are not sufficiently different to require separate 

 description. 



Before passing to a description of the second or indirect process, the 

 conditions represented in Figure 37 (Plate 6) may be noticed. Here 

 we have a condition unusual for Haminea : other eggs found on the 

 same slide as this one are in the metaphase and anaphase of the second 

 maturation. The large cell lying near the spindle of the egg is probably 

 the first polar cell. Its spindle is as perfect and clear as the one in the 

 egg. The differences between the conditions in this case and those seen 

 in the normal polar cell can be better appreciated by comparing Figure 

 37 with Figure 32 (compare also Figures 70 and 71, Plate lO). There 

 is to be seen an accessory chromosome in the egg and another in the 

 polar cell. The presence of an accessory chromosome in the polar cell 

 may perhaps help ultimately in deciphering the meaning of this 

 problematic body. Some observations made on the living egg in 1897 

 strengthen the view that this is the first polar cell. While studying the 



