56 THE MATURATION OF THE EGG OF THE MOUSE. 



the fibers become thickened at the polar ends, as he and Lams et Doorme 

 figure them. Gerlach does not agree with Sobotta that there is a central 

 spindle. While we have no evidence of the existence of a central spindle 

 like that discovered by Hermann, we agree with Sobotta that there are 

 some fibers which run from pole to pole without being attached to chro- 

 mosomes. These probably persist as a part of the interzonal filaments. 



CENTROSOMES, CIRCUMPOLAR BODIES, AND CLEAR REGION. 



No one (with the possible exception of Gerlach, fig. 2) has figured 

 the corpuscles near the poles of the spindles which we have called cir- 

 cumpolar bodies. Tafani (1889, p. 22), Sobotta (1907, p. 521, for the 

 second spindle only), and Gerlach (1906, p. 9), nevertheless, mention 

 granules at the poles, which, according to the two latter authors, form a 

 sort of mantle around the poles of the spindle and thus obscure its fibrous 

 structure. Gerlach describes them as occurring with both spindles and 

 adds that they sometimes have the form of tortuous threads, which 

 suggests to him that they may be mitochondria. 



The first impression one forms of these bodies is that they are arti- 

 facts due to improper fixation; but when one reflects that they occur 

 in eggs fixed by different methods and that they are characteristic of 

 certain stages (see p. 33), this interpretation seems unwarranted. These 

 bodies were also seen in Kirkham's preparations, although he does not 

 himself mention them. 



A study of the occurrence of these bodies brings out the fact that 

 they are characteristic of certain periods of morphological activity. 

 For example, they can be found for a short time before and during meta- 

 kinesis of the first spindle and during the early existence of the second 

 spindle when division is likely to occur as a result of semination. Con- 

 versely, they are absent during periods of morphological quiescence, 

 such as the telophase of both spindles, and when the second spindle 

 persists in the absence of semination. It will be remembered that these 

 periods of activity are very short (p. 1 6), while the quiescent periods 

 are comparatively long; therefore these bodies exist during only brief 

 periods. The question naturally arises, Are they the result or the cause 

 of the morphological changes? Unless it can be shown that they are 

 handed on from cell to cell, it seems reasonable to suppose them products 

 rather than causes of spindle activity. On the other hand, the absence 

 of typical centrosomes leads one to ask whether they may not in some 

 way fulfill the function of centrosomes, especially since they are situated 

 very close to the poles of the spindle. Such inquiries can not be answered 

 at present; these bodies, the existence of which is beyond dispute, are 

 worthy of more extensive study, and their possible relation to mito- 

 chondria should certainly be investigated further. 



Tafani, Sobotta, and Gerlach deny the regular existence of centro- 

 somes. Gerlach (1906, p. 26) saw in one case two centrioles at the pole 

 of a spindle, and Sobotta (1907, p. 524, fig. 8) figures a disk-shaped 



