SMALLWOOD: maturation of IIAMINEA SOLITARIA. 305 



during the next period the rays still converge toward a body which 

 takes a sliglitly heavier stain than do the microsomes in the cytoplasm, 

 but no centrosomal differentiation could be discovered in it (Fig. 91). 

 Apparently the centrosomes of the first cleavage spindle do not divide in 

 preparation for the next cleavage, but gradually disappear, until there 

 is no evidence of a centrosome in the egg (Fig. 97). Both centro- 

 some and sphere become indistinguishable, as in Limax agrestis 

 (Byrnes, '99). 



The appearance of the nucleus in the telophase of the first cleavage is 

 represented in Figure 95, which was constructed by superposing the out- 

 lines of the pronuclei in three successive sections. 



In the prophase of the second cleavage the chromatin passes through 

 a metamorphosis similar to that of the prophase of the first cleavage. 

 Asters arise independently in the cytoplasm ; they may lie on diametri- 

 cally opposite sides of the nucleus, or several degrees away from such a 

 diameter (Fig. 96). A large amount of material was examined in tlie 

 stages that intervene between the anaphase of first cleavage and the 

 prophase of second cleavage, but in no instance was it possible to trace 

 any connection between the asters in the prophase of the second cleavage 

 and the centrosome of the previous stage. The changes in the second 

 cleavage are a repetition of those occurring in the first. The centro- 

 somes entirely disappear, and therefore their fate cannot be followed. 



Tlie prophase of the third cleavage is shown in Plate 13, Figure 98. 

 The section passes through two of the four cells and through three of 

 the new asters. These new asters have as yet no connection with each 

 other. It looks as though the nuclear wall in the cell showing two 

 asters were drawn out in the direction of the aster which is farthest 

 from the animal pole. The cytoplasm in the vicinity of the deutoplasm 

 stains intensely, indicating a stage of great metabolic activity. This 

 phenomenon occurs at a corresponding period in the second cleavage 

 (Fig. 96), but the section does not pass through the part of the egg which 

 shows this staining property. 



The history of the nuclei and centrosomes during subsequent cleavages 

 is the same as that during the three described, the cleavage centro- 

 somes arising anew at each generation. 



Polyspermy. Occasionally two or three sperm heads are found in an 

 egg; each of these usually moves toward the animal pole until the 

 female pronucleus is reached ; then there arises a tripolar or a tetra- 



