W. G. YanName — Embryology of Eustylochus. 281 



we see that the segments are not attached by their ends only, but 

 also for a considerable distance (varying in different cases) along 

 towards their middle points, showing what their original relations to 

 each other are. 



This supports Van der Stricht's opinion that the first division is 

 in reality a longitudinal or equal one, and his suggestion that the 

 same explanations would clear up the apparently transvei'se division 

 described by Korschelt (15, 16) in Ophryotrocha. 



Van dei' Stricht describes " secondary rings " formed by the re- 

 fusion of the ends of segments already separated. It is not necessary 

 to assume such an unlikely process for the explanation of any of the 

 forms. Where we find the ends of the segments united, it is much 

 more reasonable to suppose that they have not yet separated. I 

 cannot believe that the open loops figured by Van der Stricht are 

 destined to form rings again. The only case when there is indica- 

 tion of fusion is in the form of chromosome shown in Fig. 42, where 

 the sides of the ellipse apparently do actually fuse, converting the 

 ellipse into a solid rod. 



The derived segments of chromatin which result from the form 

 of division shown in Fig. 42 are short thick rods and are really the 

 same as the loops resulting from the form shown in Fig. 41 except 

 that the two sides of the loop have come together and fused. 



The Centrosonies and Asters. 



While the aster rays of the other pole of the spindle have, as 

 already mentioned, become reduced, those of the inner pole have 

 become longer, stouter and more numerous. The centrosphere of 

 this pole has become proportionately larger, and its two centrosomes 

 have moved a little farther apart. The centrosomes seem to vary 

 greatly in size. MacFarland (20) in describing the first polar spindle 

 of Diaulula, says that the size of the central granule (" centralkorn "), 

 which undoubtedly corresponds to what I have in this description 

 called the centrosome, is dependent, within certain limits, upon the 

 extent to which the stain is extracted. Kostanecki and Siedlecki (17) 

 found much the same thing in the case of the cleavage centrosomes of 

 Ascaris, where Ftirst (5) was even led to believe that they are noth- 

 ing more than artefacts. Though I think that it would be impossible 

 to explain the distinctness, the definite form, and especially the 

 division of the black staining center of the asters in such eggs as 

 Diaulula and Eustylochus, on the supposition that the centrosome is 



