226 SOME FJWBLEMS OF CELL-ORGANIZAriOM 



distinguish in the cell, besides the " primary centres " or centrosomes, 

 secondary and tertiary centres, the latter being single microsomes 

 formed at the nodes of the network. By the successive aggrega- 

 tions of the latter may arise the secondary and primary centres as 

 new formations. Watase ('94) advocates a somewhat similar view, 

 and states that he has observed numerous gradations between a true 

 aster and such "tertiary asters" as Reinke describes. Further evi- 

 dence in the same direction is afforded by Morgan's remarkable 

 observations on the formation of "artificial asters" in vmfertilized 

 sea-urchin eggs which have lain for some time in sea-water ('96). 

 Such eggs often contain numerous asters, each of which contains a 

 body resembling a centrosome.^ Beside these observations must be 

 placed those of Richard Hertwig, on the formation of an amphiaster 

 in ripe unfertilized sea-urchin eggs (p. 159). All these observations 

 are of high interest in their bearing on the historical origin of the 

 centrosome ; but they do not prove that the centrosome of the nor- 

 mal aster ever arises by free formation. On the whole, the evidence 

 has steadily increased that the centrosome is to be classed among the 

 permanent cell-organs ; but whether it ranks with the nucleus in this 

 regard must be left an open question. 



The known facts are still too scanty to enable us to state precisely 

 what a centrosome is in a morphological sense, either as regards its 

 actual structure or its relation to other parts of the cell. In its sim- 

 plest form (Fig. 108, A) the centrosome appears under the highest 

 powers as nothing more than a single granule of extraordinary 

 minuteness which stains intensely with iron-haematoxylin, and can 

 scarcely be distinguished from the cyto-microsomes except for the 

 fact that it lies at the focus of the astral rays. In this form it 

 appears at the centre of the young sperm-aster in various animals 

 — for example in the sea-urchin (Boveri), iji Chcetoptcnis (Mead), 

 and in Nereis? In almost all cases, however, the centrosome after- 

 wards assumes a more complex structure and becomes surrounded 

 by certain envelopes, the relation of which, on the one hand, to the 

 centrosome and, on the other hand, to the astral rays have not yet 

 been fully cleared up. 



Boveri, whose observations have been confirmed by Brauer, Hacker, 

 and others, described the centrosome in the cleavage-asters of Asearis 

 as a small sphere containing a minute central granule ; and Brauer's 

 careful studies on the spermatogenesis of the same animal showed 



1 I have had the privilege of examining Professor Morgan's preparations, and can contirm 

 his statement that these eggs contain but a single nucleus and hence are not polyspermic. 



2 This appearance is not due to the shrinkage of a larger and more complex structure, as 

 some authors have suggested; for in Nereis such a structure — i.e. the centrosphere — is 

 afterwards developed around the centrosome. 



