456 WESLEY R. COE, 



diminution in size. Following these are the eggs of Chaetopterus (21), 

 of Ophryotrocha (13), and of some Echinoderms in which the sperm- 

 asters develop very early, but which are not described as decreasing 

 in size before the formation of the cleavage-spindle. Then come the 

 eggs of Toxopneustes (34), and of Thalassema (7), where the sperm- 

 asters appear early and develop to a very considerable size, but never- 

 theless become very much smaller and less conspicuous after the 

 germ-nuclei have come together ^). After these we must place the 

 eggs of Physa (15) ^), for here the sperm-asters after becoming very 

 large and conspicuous degenerate to such an extent that only a "very 

 few, exceedingly delicate fibres" remain. Those of Cerebratulus follow 

 next. Here the sperm-asters increase in size until they extend 

 throughout the whole body of the cell, but at the time of fusion of 

 the germ-nuclei they degenerate completely. The peripheral portions 

 of their fibres, however, may be followed, as stated above of Pleuro- 

 phyllidia (18)^), Prostheceraeus (12) etc., where the sperm-asters 

 degenerate soon after their formation, so that for a considerable period 

 the egg is without trace of aster -fibres. Yet in all of those cases 

 where the sperm-asters disappear, and their centrosomes become lost 

 among the other granules of the cell we are justified in believing that 

 the sperm-centrosomes nevertheless retain their identity, and later 

 reappear in the cleavage-asters. The reasons for such a belief are, 

 it is true, based quite as much on the analogous phenomena which 

 are found in other eggs where the sperm-centrosomes do not disappear, 

 as on the evidence which we obtain from the actual preparations in 

 question. 



The difficulty of determining this point with absolute certainty 

 in other eggs is well shown by the fact that two so able investigators 

 as Wheeler and Kostanecki, working upon the same species of 

 animal, have arrived at diametrically opposite conclusions in regard 

 to the origin of the centrosomes found in the cleavage-spindles. The 



1) Wilson finds that when the cleavage-nucleus becomes elongated 

 the sperm-asters, which were previously very massive, become so much 

 reduced that their fibres are "scarcely apparent". 



2) A comparison of the figures given by Kostanecki & Wiek- 

 zEJSKi for Physa with those which MacFakland gives for Pleur ophyllidia 

 convinces me that there is not nearly so much difference in the figures 

 for the two species, as there is in the interpretation which these authors 

 give of them. This is especially noticeable in the sequence iu which 

 the figures are arranged. 



