BIGELOW: NUCLEAR CYCLE OF GONIONEMUS MURBACIIII. 353 



and now (Fig. HO), instead of centring about the sperm body as a 

 whole, are directed toward a definite point at some distance from it. 

 Although the aster is often very distinct, I have never been able to dis- 

 tinguish any definite body or granule at its centre which might be con- 

 sidered an undoubted centrosome. There is, however, in all cases a 

 central, ill-defined, deeper staining region or sphere, in which the radia- 

 tions are lost, and this may perhaps represent a centrosphere. l'>y the 

 time that the second maturation spindle has reached the anaphase, the 

 sperm nucleus is as a rule deep in the substance of the egg, and in 

 almost all cases the sperm aster which accompanies it has divided, the 

 two daughter asters lying, not, as might be expected, in advance of it in 

 its line of migration toward the egg nucleus, but on either side of it, as 

 is shown in the particularly clear example represented in Figure 141. 

 The time relations of the different steps in fertilization show great in- 

 dividual variation, and in some cases this division of the sperm aster 

 takes place as early as the metaphase of the maturation division, while 

 in others the sperm nucleus may be accompanied by only a single centre 

 of radiation until it is almost in contact with the egg nucleus. Similarly, 

 the changes in the sperm nucleus itself may take place earlier or later ; 

 but in any case, between the time of the division of the sperm aster and 

 the re-formation of the egg nucleus, the sperm nucleus grows much 

 larger, attaining to two or three times its previous bulk (compare Figs. 

 14T and 142). At the same time the sperm nucleus, which was previ- 

 ously homogeneous, now lies in a transparent, unstained area in the egg 

 cytoplasm, loses much of its affinity for stains and grows distinctly 

 granular. 



After the metaphase the remainder of the second maturation mitosis 

 is rapidly accomplished. The second polar cell is formed by the usual 

 process of constriction, and the portion of the spindle figure remaining 

 in the egg and its aster disintegrate and entirely disappear. The egg 

 nucleus, now containing only half the somatic number of chromosomes 

 (probably twelve) acquires a membrane. Then the chromosomes disin- 

 tegrate into their component microsomes, leaving the nucleus in a 

 reticular condition in which the chromatin is so finely diffused as to give 

 it a granular appearance (Fig. 144). This stage is essentially like that 

 following every adult mitosis in Gonionemus, except that no nucleolus 

 is formed. 



The egg nucleus now commences to move centrally toward the sperm 

 nucleus, while the latter once more takes up its migration, with the 

 result that the two nuclei come together in a somewhat eccentric posi- 



VOL. XLVUl. — no. 4 23 



