438 WESLEY R. COE, 



appears to be due in part to the actual coalescence of smaller asters." 

 Mead is convinced that the "centrosomes in the Chaetopterus ovum 

 arise by a modification of the cytoplasmic reticulum." In this view 

 he is supported by Watasé's observations on the egg of Macrobdella 

 (29) and by Reinke's descriptions (22) of the structures found in 

 certain of the tissue cells of the larval salamander. It seems to me 

 not impossible, however, that in Chaetopterus a pair of actual centro- 

 somes may appear near the nuclear membrane exactly as has been 

 described in other animals, and that these centrosomes may form the 

 centres of asters at first indistinguishable from the "secondary asters", 

 but which are, nevertheless, fundamentally different as to their actual 

 nature. In a very few preparations of the eggs of Micrura I have 

 observed indications of somewhat similar "secondary asters", but these 

 were in eggs which were so manifestly immature that I have con- 

 sidered them to be pathological structures. 



Both KosTANECKi (17) and Wheeler (32) have found in Myzo- 

 stoma that at the first appearance of the centrosomes they lie very 

 near together, and that they gradually move apart. Kostanecki (17) 

 describes them as being connected by a very distinct spindle which 

 continually increases with the growth of the asters. Wheeler (32) 

 finds at one side of the germinal vesicle "a pair of centrosomes con- 

 nected by a delicate achromatic bridge" with aster-fibres radiating 

 about them. "The two asters thus established very soon move some 

 distance apart, and the centrodesmus instead of giving rise to a 

 central spindle as in many other cases which have been described, is 

 torn asunder." Neither Wheeler nor Kostanecki has figured any 

 of the intermediate stages. If such a division of the centrosome occurs 

 in Cerebratulus, as it very probably does, it must take place at a 

 very early period, and before the asters have been established about 

 them for even when the radiations are most minute, as shown in 

 Figs. 2, 3, the asters are very widely separated. My observations 

 agree almost exactly with those of Griffin (7) on Thalassema. 

 Stauffacher (26) found a somewhat similar state of things in Cyclas. 

 KoRSCHELT finds in the Annelid Ophryotrocha (13) that the aster 

 found near the germinal vesicle develops to a considerable size before 

 division. After dividing, the two halves pass to opposite sides of the 

 nucleus without any indentation of the nuclear membrane, or affecting 

 it in any manner. The observations of Klinckowström (12) on the 

 Planarian Prostheceraeus agree in the main with those recorded above, 

 but in a single instance he has seen and figured a centrosome which 



