280 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



stain, and only the primary rays extend through it to the centrosorue 

 (Plate 4, Figs. 20, 21). This region will he referred to henceforth as 

 the medullary layer (st. med.). It lies entirely outside the centrosonie, 

 and is not to be confounded with the terra " medullary layer " as used by 

 Conklin (: 01) and myself (: 01). The medullary layer in the sense in 

 which I now use the term is differentiated from the " cortical layer " 

 (see p. 267). In Figure 19 is shown a condition transitional between 

 that of the cortical layer seen in Figures 13, 16 (Plate 3), and that of 

 the layer after the differentiation from it of the medullary layer (st. med.) 

 as seen in Figures 21 et seq. (Plate 4). Here (Fig. 19) the medullary 

 layer takes a rather heavy plasma stain, and the secondary rays seem to 

 terminate at its outer limit. In the following stages the medullary layer 

 is clear, taking a very light stain, and it has no regular and sharply de- 

 fined outer limit. In general form and extent it corresponds to the 

 following conditions observed in other animals : for Diaulula MacFarland 

 ('97) shows in Figures 29-32 (Taf. 20) a region which corresponds to 

 the medullary layer in Haminea, and Lillie (: 01, Plate 24, Fig. 9) 

 figures for Unio a similar condition ; the same region is named by Conk- 

 lin (: 01, Figs. A, 3, 3-5, and B, 3) cortical ; and Boveri (: 01, Taf. 5, 

 Figuren 56-58) in describing it uses the expression "Zone medullaire," 

 a term first used by Van Beneden et Neyt ('87). 



The form of the medullary layer in Haminea varies considerably, 

 owing to the extent of the encroachment of the cortical layer. When 

 the outer pole of the mitotic figure reaches the periphery of the egg, the 

 medullary layer becomes indistinguishable. Some of the changes in- 

 volving variations in the outline of the medullary layer are illustrated 

 in Figures 21, 23, 25, 27, 29 (Plates 4, 5). 



Chromosomes. A cross-section of the spindle at the equatorial plane 

 in the mctaphase is represented in Figure 24 (Plate 4) ; there are sixteen 

 chromosomes, and between them can be seen the cross-sections of fibres, 

 which appear as small dots. 



While the egg remains in the ovotestis the chromosomes retain the 

 tripartite form, which seems to be a resting condition ; but as soon as it 

 is laid, these irregular bodies tend to fuse into straight rods of nearly 

 uniform calibre. A typical condition is seen in Figure 22 (Plate 4), 

 where the chromosomes are, in the main, straight rods, though slight 

 irregularities are to be noticed in the outline of two of them, — the large 

 chromosome l^'ing on one side of the spindle, which has probably resulted 

 from the fusion of two or more, and one of the narrow ones. This large 

 chromosome would doubtless have become separated into two or more 



