THE CENTROSOME 309 



referred to at page 215, finds that after treatment with magnesium 

 chloride unfertilized sea-urchin eggs (Arbacia) may give rise to perfect 

 Pluteus larvae a result which if well founded seems to place the 

 new formation of true centrosomes beyond question. 



Taken together, these researches give strong ground for the con- 

 clusion that true (i.e. physiological) centrosomes may arise de novo 

 from either the cytoplasmic or the nuclear substance and may play 

 the usual role (whatever that may be) in mitosis. If this cdnclusion 

 be sustained by future research, we shall no longer be able to accept 

 Van Beneden's and Boveri's conception of the centrosome as a per- 

 sistent organ in the same sense as the nucleus ; but on the other hand 

 we shall have gained important ground for further inquiry into the 

 nature and source of that power of division which is so characteristic, 

 of living things and upon which the law of genetic continuity rests. 



Morphology of the Centrosome. In its simplest form (Fig. 152, 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 always appears at the centre of the very 

 young sperm-asters during fertilization (Figs. 97, 99), in the early 

 phases of ordinary mitosis (Figs. 27, 32), and in some cases also in the 

 resting cell, for example, in leucocytes and connective tissue corpuscles 

 (Figs. 8, 49), where, however, it is often triple or quadruple. In the 

 course of division the centrosome often increases in size and assumes 

 a more complex form, becoming also surrounded by various structures 

 involved in the aster-formation. The relation of these structures to 

 the centrosome itself has not yet been fully cleared up and there 

 is still much divergence of opinion regarding the cycle of changes 

 through which the centrosome passes. It is, therefore, not yet possi- 

 ble to give a very consistent account of the centrosome, still less to 

 frame a satisfactory morphological definition of it. 



It is convenient to take up as a starting-point Boveri's ('88) account 

 of the centrosomes in the egg of Ascaris, supplemented by Brauer's 

 ('93) description of those in the spermatocytes of the same animal. 

 During the early prophases of the first cleavage Boveri found the 

 centrosome as a minute granule which steadily enlarges as the spin- 

 dle forms, until shortly before the metaphase it becomes a rather large, 

 well-defined sphere in the centre of which a minute central granule or 

 centriole appears (Fig. 152, B, C). From this time onward the cen- 

 trosome decreases in size until in the daughter-cells it is again reduced 

 to a small granule which divides into two and goes through a similar 

 cycle during the second cleavage and so on. The centrosome is 

 at all stages surrounded by a clear zone (" Heller Hof ") in which 



