326 WILSON AND MATHEWS. [Vol. X. 



passing through the former spindle-axis) into two halves which 

 place themselves at opposite poles of the nucleus, and this is 

 followed by a division of the surrounding blue mass (Fig. 4, C). 

 The central masses (still staining red and surrounded by the blue 

 granular mass, traversed by faint rays) persist in this position 

 throughout the 2-celled stage and form the centers of the second 

 cleavage-spindles, which resemble the first in all respects. 



It is clear from these facts that the central reticular ery- 

 throphilous sphere is identical with an "attraction sphere," but 

 it contains no differentiated centrosome or even a "central 

 group." It will now be shown that the arcJioplasin-spheres are 

 derived by direct descent from the sperm-arc hop lasm {and ulti- 

 mately from the middle-piece) without the participation of a 

 visible '^ ovo centred 



2. Entrance of the Spermatozoon and Copulation of the Pronuclei. 



The head of the spermatozoon (Fig. 2, A) is lance-shaped, 

 the nucleus staining intensely blue, while the middle-piece is 

 red. (There is also a very minute erythrophilous body at the 

 tip, easily demonstrable in fresh material, but not visible after 

 fixation.) It enters the ^gg point first (Fig. 2, B), but almost 

 immediately afterwards rotates through about 180°, so that its 

 base is directed inwards (Fig. 2, B-F). The middle-piece 

 cannot be distinguished in Toxopneustes at first {cf., however, 

 Arbacid) since it stains like the surrounding cytoplasm. Dur- 

 ing the rotation, however, a small aster appears at or near the 

 base of the nucleus (Fig, 2, D), and this is undoubtedly de- 

 rived from the middle-piece. The aster rapidly grows as the 

 sperm advances, its rays finally traversing nearly an entire 

 hemisphere of the ^gg. The central body of the aster is occu- 

 pied by an ill-defined granular mass, staining red, while the 

 astral rays are faintly blue. The sperm-aster therefore agrees 

 on the whole with the cleavage-aster, but the central mass does 

 not yet show a definite reticular structure, and the color-con- 

 trast with the rays is less marked. 



During its inward progress the aster is in advance of the 

 sperm-nucleus, and as a rule lies somewhat to one side of it. 



