No.2.) CENTROSOMES IN THE ANNELID EGG. 185 
of the cell, as the nucleus and the cytoplasm. According to 
the other view (2) the centrosome is a derivative structure, 
arising by the modification of some preéxisting element in the 
cell, as the chromosome, nucleolus, or the cytoplasm.’ 
The theory that the centrosome is a permanent and ultimate 
organ of the cell finds direct support in the observations of a 
number of investigators, which prove beyond doubt that the 
centrosome is capable of self-division and growth, and that it 
may persist from one generation to another. 
In Chzetopterus the centrosomes can be seen distinctly in 
every phase of mitosis, and are always surrounded by an aster, 
even in the resting stage. On the other hand, in certain 
animals, the centrosomes are not always demonstrable in the 
asters. Sometimes both the asters and the centrosomes vanish 
for a while and then reappear (Wheeler, Lillie, MacFarland, 
etc.). It is conceivable, however, that, though invisible, the 
centrosomes may be present in the cell, for they have been 
found to lie naked in the cytoplasm, bereft of rays (Heiden- 
hain, '93). 
To maintain that the centrosomes are absent, simply because 
they are not demonstrable, is, of course, to base an assumption 
upon negative evidence, a procedure especially dangerous when 
applied to the centrosome, inasmuch as this structure is, at 
best, very minute, and comparatively difficult to demonstrate, 
even when its exact position is indicated by the presence of an 
aster. 
But, admitting that the centrosome is “a persistent morpho- 
logical element having the power of growth, division, and 
persistence in the daughter cells,’ and even admitting that it 
exists incognito, it remains to be proved that it arises only by 
division from a preéxisting centrosome and that it does not 
cease to exist as a morphological element in cells which 
subsequently possess centrosomes. 
Watasé has pointed out that the transition between centro- 
somes and other structures of the cytoplasm should be sought, 
not among the most highly developed typical centrosomes, such 
as are to be found in mitosis, but among structures — centro- 
somes or homologues of centrosomes — which are less persistent 
