No. I.] POLYCHOERUS CAUDATUS. 93 



indicates the existence of an "attraction " or "entrance cone," 

 though the structure of this part of the ovum differs very 

 slightly from the rest of it, except that it stains a little less 

 readily. Hence the manner, as well as exact point of entrance 

 with reference to the future plane of all cell cleavage, remains 

 undetermined. The fact that, shortly after entrance has been 

 effected, the sperm always lies near the pole opposite to that 

 at which the polar globules are formed seems, however, to 

 indicate that it has entered on the lower side of the ovum. 

 At the time when the first maturation spindle begins to 

 form, the sperm may generally be found at a distance from 

 the surface of less than one-fourth the egg diameter, and the 

 entrance cone has entirely disappeared. By this time the 

 sperm has increased enormously in diameter and is surrounded 

 by a peculiar substance (Fig. 23), which suggests in its appearance 

 a ball or snarl of thread, and which stains but slightly. This 

 substance must have been differentiated from the surrounding 

 cytoplasm while the sperm is moving toward the egg center. 

 While this growth of material about the sperm to form the 

 complete male pronucleus takes place, that portion of the 

 pronucleus which entered as sperm head increases very 

 markedly in diameter (Figs. 24 and 25), and at the same time 

 decreases in length as if it were melting away in the substance 

 built up around it by itself. 



As yet no rays or aster are formed in connection with it, 

 although it progresses steadily toward the center of the 

 ovum. This is of interest, for in some cases the movement 

 through the cytoplasm has been attributed to the action of 

 the aster and in others to the movement of the sperm itself. 

 Since in this case no aster exists and the sperm is completely 

 imbedded in a substance built up by itself from the cytoplasm 

 and which moves with it, as if it were an integral part 

 of it, the translation must be due to other forces. When 

 close to the center of the ovum the remnant of the sperm 

 head is represented by a crescent-shaped dark-staining mass 

 (Fig. 25), still surrounded by a fibrous snarl of thread-like 

 substance. Then the chromatin begins to increase by the 

 breaking up of this head, until the whole pronucleus is filled 



