58 KARYOKINESIS. 



ation division of Unio is composed of several large granules into which rays are 

 inserted. In the metaphase these granules subdivide, and some of the fragments 

 are distributed in the form of a sphere, the "inner sphere;" "one of the granules 

 remains behind as the centrosome of the new inner sphere," but " a large part of 

 the centrosome granules is changed into the red-staining substance of the sphere." 

 In the anaphase the granules of the inner sphere, together with the peripheral 

 accumulation of its ground substance, fuse into a continuous membrane. " The 

 centrosomes are united to the membrane of the inner sphere by a few irregular 

 threads which are not part of the sj-stem of radiations." Within the sphere the 

 daughter centrosomes and central spindle arise. 



Lillie emphasizes the fact that the inner sphere is not the centrosome, and he 

 says that Boveri's "centrosome" is reall}' the inner sj)here, while his "centriole" is 

 the real centrosome. He also holds that MacFarland's " centrosome" is really the 

 inner sphere. 



Lillie's conclusions seem at first sight to be very different from any of those 

 mentioned above, and j-et on consideration it will be seen to be rather a difference 

 of terms than of facts. His "inner sphere" is undoubtedly homologous with Boveri's 

 centrosome, his "centrosome" with the central corpuscle or "centriole" of Boveri. 

 As to the genesis of these parts, I have never been able to observe the formation of 

 the "inner sphere membrane" from granules derived from the central corpuscle as 

 Lillie has done, nor have I observed the fragmentation of the central corpuscle and 

 the transformation of these granules into the ground substance of the centrosome 

 (substance of medullary zone), though in earh-- phases the centrosome of Crepidula 

 is irregular in outline, as if composed of closely connected granules. In all cases 

 which I have observed the central corpuscle enlarges but does not fragment ; its 

 substance accumulates peripherall}^ and forms a continuous membrane which subse- 

 quently is transformed into a layer of granules. If the central corpuscle of Unio 

 were to remain a single structure, and were to continually expand, the result would 

 not be unlike my observations. The one critical point in the comparison of Lillie's 

 observations with those of other investigators, is to determine whether the whole of 

 his " inner sphere " is derived from the central corpuscle ; if it is, the differences are 

 only matters of detail. It should be remembered that, according to Lillie, the inner 

 sphere is itself a structure which sooner or later disintegrates and passes into the 

 outer sphere or c^'toplasm, and that it should disintegrate at different stages in 

 different eggs is quite possible.' 



In a later paper on the subject, Lillie ('99) says that the inner sjjhere enlarges 

 very rapidly after the formation of the second polar body, and its substance gradually 

 merges with the general cytoplasm. Its interior is occupied by the vesicular sphere 

 substance at the nodes of which are deepl}- staining granules. In this respect there 

 is considerable difference between Unio and Crepidula, for in the latter the inner 



^ I ought to add that I have had the pleasure of seeing Lillie's beautiful preparations, and they 

 leave no ground for doubting the accuracy of his observations. Professor Lillie also personally assures 

 nie that he is quite convinced that the whole of the "inner sphere" is derived from the central cor- 

 puscle. (See also his recent work, 1900, p. 242. J 



