42 THE STRUCTURE OF CELLS 



But in the numerous instances in which amitosis occurs (as in 

 budding), the " centrosome " does not divide, and the nucleus of 

 the freshly-budded cell possesses no such structure. Soon after the 

 complete formation of the bud, however, a dense spot is formed 

 within the nucleus, and is then extruded into the cytoplasm, where 

 it continues to function as a centrosphere. The evidence, as drawn 

 from a study of the lower organisms, seems to point strongly in 

 favour of a nuclear origin for the centrosome apparatus in many 

 cases, although the simplest examples cited on a previous page also 

 indicate that, in others, such a structure might be coeval with, if 

 indeed not actually antecedent to, the primary differentiation of a 

 true nucleus. 



In a considerable number of cases it seems at least clear that 

 the body or the substance which stimulates and brings about the 

 division of a nucleus is derived from the nucleus itself, even though 

 it may migrate into the cytoplasm, where it may continue to exert, 

 under appropriate conditions, that influence on the nucleus which 

 culminates in division. Thus, in diatoms the centrosphere is 

 found in the cytoplasm, just as it exists in many metazoon cells. 

 The actual location of the centrosome does not necessarily, however, 

 settle the question as to its real origin, and it may indeed assume 

 an intra- or extra-nuclear position in closely allied forms ; as, for 

 example, in the two varieties of Ascaris megalocepJiala, being 

 situated within the nucleus in the variety univalens, and outside it 

 in the variety bivalens. 



A remarkable side issue has been introduced into the contro- 

 versy as to the phylogenetic origin of the centrosome by a considera- 

 tion of the peculiar nuclear apparatus which is met with in most 

 ciliata and suctoria. These organisms, with a few possible exceptions, 

 possess a mega- and a micro-nucleus, the former presiding over the 

 somatic life and divisions of the animal, the latter only becoming 

 prominent during the phases of sexuality. Some writers have 

 sought to derive, phylogenetically, the centrosome from the micro- 

 nucleus, whilst they see in the meganucleus the representative of 

 the metazoon nucleus. But quite apart from the fact that, as 

 Schaudinn pointed out, centrosomes appear in the much simpler 

 heliozoa, the fact that the micronucleus alone divides mitotically, 

 whilst the meganucleus always does so by amitosis, seems a serious 

 difficulty in accepting such an interpretation. Moreover, the macro- 

 nucleus itself springs from the micronucleus after each sexual act, 

 and only persists till the close of the sexual cycle, at which period 

 it totally disintegrates, and thus suffers somatic extinction. It 

 would certainly appear that at any rate it is useless to look to such 

 a source for the origin of the centrosome, which really seems to 

 rest on no better basis than a purely fanciful comparison. 



A consideration of the maturation processes which obtain in 



