MUSEUM OF COMPARATIVE ZOOLOGY. 245 



figures made prior to the time (1873) when they were shown to be in- 

 timately connected with the nuclear changes will be historically reviewed. 

 In order to study the changes which take place in the nucleus during 

 cell-division, it will also be desirable to review late studies and opinions 

 on the nature of the quiescent nucleus, — if we may speak of an appar- 

 ently less active condition as a quiescent state. This will necessitate a 

 second digressioa from the main topic, now under consideration. 



a. Asters. 

 • Stellate arrangements of the protoplasm surrounding the nucleus were ob- 

 served long ago, but it is very questionable if many of these earlier observations 

 were really made on the stellate figures which accompany cell-division. Es- 

 pecially doubtful are those descriptions in which a distinct nuclear structure is 

 made the centre of an extensive radiation in the protoplasm. For the fully 

 formed, conspicuous nucleus does not in most cases correspond to the stage 

 in which the radial arrangement is prominent. In fact, with the completion 

 of the nucleus, the radiate structure usually vanishes altogether. The repre- 

 sentation of rays about a nucleus is not enough, then, to warrant the conclusion 

 that an author has observed the astral figures which accompany division. To 

 accept such as sufficient evidence is to ignore an almost constant relation of the 

 astral centre to the nucleus. 



One of the earliest and most striking cases of such a radial phenomenon is 

 that described and figured by Carus ('32, pp. 44, 45, Taf. II. Figs. 3, 10, 11). 

 It is true Carus regards, though erroneously, the embryos (Unio) figured as 

 presenting abnormal conditions resulting from death, yet the nucleus and the 

 radial structure of the surrounding protoplasm are so clearly shown that at first 

 one would not hesitate to pronounce his figures to be those of veritable asters. 

 So strong as the resemblance is, it must, however, be admitted that it is at 

 best only a resemblance, and not a true aster. The latter does not exist when 

 the nuclei present the appearance reproduced in Carus's figures. Flemming 

 does not hesitate, after a study of the same objects, to declare the radiation in 

 this case to be purely the result of a play of fancy. 



The figures accompanying the work of Grube on Clepsine ('44, Taf. III. 

 Figs. 11, 12) would lead one to suspect that he might have seen the radial ar- 

 rangement of the protoplasm in the periphery of the nuclei, his " Wandungs- 

 kugeln"; but I do not find that he makes any mention of it in the text, and 

 am by no means sure that the radiate lines may not be due to something 

 entirely dijfferent, possibly to fissures caused by excessive or unequal hardening 

 of the eggs. At least, his figures do not present as acceptable evidence that 

 such phenomena were seen as do those which Derbes ('47, PI. V. Figs. 4, 6) 

 executed three years later in giving the embryology of the urchin (Echinus es- 

 culentus). Derbes's first-mentioned figure is that of the egg after the disappear- 

 ance of the germinative vesicle,* but before the first segmentation. From the 



* Derbes, it is true, did not regard his " sphere moyenne " as a germinative vesicle. 



