86 NICHOLS — SPERMATOGENESIS ONISCUS ASELLUS LINN. [April 4, 



decolorized, a longitudinal split is evident and likewise a division 

 of the chromosome into chromomeres. If the chromosome is of 

 the second type and seen from above, two of the chromomeres will 

 be seen longitudinally split (Fig. 46; cf. Fig. 53). An end view of 

 a chromosome of the first type shows simply a single chromomere 

 longitudinally split (Fig. 59). 



Linin connections between the chromosomes are much more evi- 

 dent than in the spermatogonia, and they can be seen to extend 

 from the sides as well as the ends of the chromosomes. 



With regard to the origin of the first maturation spindle-fibres it 

 is difficult to be certain, but they appear to arise, at least in part, 

 from within the nucleus. The centrosome is not evident until a 

 rather late prophase (Figs. 39, 40, 43). In many cases it lies 

 within a more densely staining mass of cytoplasm of ill-defined out- 

 line applied close to the nucleus (sphere substance, idiozome of 

 Meves) (1898) (Figs. 39^, 43). This is not, however, invariably 

 the case, as may be seen from Figs. 40, 39^^, where the centrosomes 

 lie freely in the cytoplasm. Fig. 38^ perhaps represents an early 

 stage in the development of the sphere substance. In the two 

 adjacent cells (Fig. 38^ and 38^:) are shown rounded bodies of a 

 dull tint lying within clear vacuoles. I met these in but one 

 preparation and am unable satisfactorily to interpret them. ^The 

 division of the centrosome and the formation of the spindle is 

 shown in Figs. 46, 47, 48, 52. The centrosomes and spindle-fibres, 

 as well as the polar radiations, are more prominent than in the 

 spermatogonic spindles. During this time the sphere substance 

 disappears. 



In the equatorial plate the chromosomes become arranged with 

 the longitudinal split parallel to the axis of the spindle in the case 

 of chromosomes of the first type, but at right angles to it or nearly 

 so in the case of chromosomes of the second type (Figs. 49, 50 and 

 53). In Figs. 55 and 56 are represented pole views of both types 

 of chromosomes. It may be gathered from these, as well as from 

 the figures of the prophases, that chromosomes of the second type 

 are not nearly so numerous as those of the first nor so numerous as 

 those of the intermediate type. 



From what has been said with regard to the origin of the chro- 

 mosomes, it will be seen that in the metaphase the bivalent chromo- 

 somes are separated into their univalent components, and conse- 

 quently \}ci^ first division is one of reduction. 



