390 



SCIENCE. 



[N. S. Vol. V. No. 114. 



lying in the egg substance is, at this time, 

 perfectly naked, i. e., unaccompanied by 

 archoplasm or radiation of any sort. As 

 the sexual nuclei approach each other, 

 radiations appear around the centrosome in 

 the archoplasm ; or these radiations appear 

 first after the nuclei have met. There can 

 be no doubt in either case that the centrosome 

 around ivhich they apixar is the egg center, see- 

 ing that it lies within the archoplasm, 

 which always accompanies the egg nucleus. 

 The centrosome then divides in two, form- 

 ing the amphiaster of the first cleavage. 



The egg and sperm nuclei never fuse to 

 form a single vesicular cleavage nucleus, 

 but each forms its own group of sixteen 

 chromosomes. 



The study of the earlier stages shows, 

 that the sperm head is accompanied soon 

 after its entrance by a comet-like aster, 

 with a minute centrosome. This centrosome 

 divides and forms an amphiaster, which entirely 

 disappears in the late anaphase of the first matu- 

 ration spindle. The sperm centrosomes never be- 

 come functional again. But a supernumerary 

 central aster appears near the center of the 

 egg during the metaphase of the second 

 maturation spindle. This aster also disap- 

 pears during the anaphase of the same 

 spindle. 



Thus the mode of fertilization in TJnio 

 agrees with that in Myzostoma, in so far as 

 the centers of the cleavage spindles are de- 

 rived from the ovum, but differs from it in 

 as much as the spermatozoon in Unio 

 brings in a centrosome, whereas the sper- 

 matozoon of Myzostoma does not introduce 

 a centrosome into the ovum. Unio is thus 

 in a certain sense intermediate between 

 Myzostoma and those forms in which the 

 sperm centrosome forms the active centers 

 of the cleaving ovum. 



Centrosome and Middle-jnece in the Fertiliza- 

 tion of the Egg. E. B. Wilson. 

 In an earlier paper the author had de- 



scribed the sperm-aster in Toxopnemtes as 

 arising about the middle-piece of the sper- 

 matozoon as a center. Within the cen- 

 tral mass thus formed no constant central 

 granule could be found ; the conclusion 

 was therefore drawn that the middle-piece 

 as a whole must be identified as the centro- 

 some. Later studies on material differently 

 fixed show that this conclusion was erro- 

 neous. In eggs fixed in picro-acetie and 

 weak sublimate-acetic (1-5 per cent, acetic) 

 the middle-piece stains intensely black and 

 its entire history can be accurately followed 

 in sections. As the sperm nucleus moves 

 inward the middle-piece separates from the 

 nucleus, is left behind, and finally breaks up 

 and degenerates. The astral rays are thus 

 found to focus at a point lying at the base 

 of the nucleus, between it and the middle- 

 piece. At this point is an extremely minute 

 intensely shining granule, which undoubt- 

 edly is the centrosome, as described by 

 Boveri, von Eath, Hill and Kostanecki. 

 The centrosome occurs in the same form in 

 Arbacia and Asterias. In some cases the 

 sperm-aster and centrosome move away 

 from the nucleus before the latter has sep- 

 arated from the middle-piece. These facts 

 demonstrate that the middle-piece proper 

 is not the centrosome, and that the latter 

 is an infinitesimal granule which lies either 

 inside the middle-piece or between it and 

 the nucleus. 



In Arbacia the sperm centrosome can be 

 traced continuously though the first cleav- 

 age into the 2-cell stage, as in Chwtojyterus, 

 Thalassema and Physa ; and precisely as in 

 those forms in the late anaphases each 

 cleavage centrosome, after doubling, gives 

 rise to a daughter-amphiaster and central 

 spindle which are, however, of extraordinary 

 minuteness. In Toxopneustes, after exactly 

 the same treatment, the result is apparently 

 different, agreeing in substance with the 

 author's earlier studies and with the ac- 

 counts of Boveri and Keinke. In the ' pause,' 



