330 WILSON AND MATHEWS. [Vol. X. 



unbroken descent from the central mass of the sperm-aster with- 

 out visible participation of an egg-center. There is no centro- 

 some save as an artefact. The sperm-aster is derived from the 

 middle-piece, which like the archoplasm-sphere at every stage is 

 erythrophilons. 



II. The Pseudo-Quadrille in Dispermic Eggs of Toxo- 



PNEUSTES. 



The most careful study of sections at every stage of the 

 normal fertilization gives absolutely no evidence of the exist- 

 ence of an egg-aster, an egg-centrosome, or a copulation of 

 centrosomes. In double-fertilized eggs, however, a conspicuous 

 pseudo-quadrille is enacted that affords important indirect evi- 

 dence. Polyspermy may take place both in ripe and unripe 

 eggs. In the latter case (eggs with germinal vesicle) the 

 sperm-head penetrates only a short distance, does not unite 

 with the egg-nucleus, and no aster is developed. In the former 

 case (after extrusion of the polar bodies) each sperm-nucleus 

 is accompanied by an aster that ultimately divides into two, 

 forming a sperm-amphiaster, as is normally the case in Asterias 

 {vide infra). In dispermy (very common in Toxopneustes) both 

 sperm-nuclei copulate with the egg-nucleus, the three nuclei 

 fusing completely together. Meanwhile, a symmetrical tet- 

 raster (Fig. 4, D), is developed by union of the two sperm- 

 amphiasters, and the &gg ultimately divides into four at the 

 first cleavage 1 (as described by Hertwig and Driesch). 



It has already been suggested that Fol may possibly have 

 been misled by such cases of dispermy. Waiving this point, 

 however, it may be pointed out that if an egg-centrosome were 

 present in Toxopneustes the tetraster of dispermic eggs would 

 result from the combination of six centers instead of four, and 

 it is hard to conceive how a symmetrical division (such as 

 actually occurs) could take place. Beside this indirect nega- 

 tive evidence, should be placed the additional fact described 

 above, that in normal monospermic fertilization the sperm may 



1 This process was repeatedly observed in the living egg, and nearly every stage 

 of it has been carefully studied in sections. 



