February 5, 1897.] 



SCIENCE. 



233 



When the last traces of the paranucleus have 

 vanished the cytoplasm presents a nearly uni- 

 form appearance throughout. The reticulum 

 is characterized at this stage by the decidedly 

 granular composition of the fibrils and the 

 circular appearance of the meshes as seen in 

 section, and also by the uniform distribution of 

 yolk throughout the egg. 



Although the structure of the reticulum, the 

 peripheral egg membrane (pellicle) and nuclear 

 membrane are beautifully clear and easily 

 demonstrable, there is as yet no trace of anything 

 suggesting a centrosome. 



Soon after this, as the egg grows larger, the 

 cytoplasmic threads show a tendency to an 

 arrangement in straight lines rather than curves, 

 so that the outlines of the meshes are polygonal,^ 

 rather than circular. Eggs which have reached 

 this stage in development, when placed in sea 

 water, continue to develop as far as the forma- 

 tion of the first maturation spindle. The 

 tendency of the fibrils of the network toward 

 straightening out becomes accentuated, so that 

 many of them extend in straight lines for a dis- 

 tance several times the diameter of the single 

 meshes. Moreover, these longer fibrils radiate 

 from common centres, and in this way there 

 arises in the cytoplasm a number of miniature 

 asters. At first only two or three rays may be 

 seen ; then they increase in number and length 

 at the direct expense of the remaining network. 

 The Aster formation continues until a climax is 

 reached, when one can count no less than seventy- 

 five distinct asters scattered about through the 

 cytoplasm in the vicinity of the meshes. (Many 

 of them are half way between nucleus and 

 periphery of the egg.) These asters repel the 

 yolk as do those of the caryokinetic spindle. 



The period of development characterized by 

 multiple asters is not of long duration. Two of 

 the asters gain predominance over the others 

 in point of size, and continue to grow larger, 

 while the others gradually evanesce. The two 

 larger asters I will call primary, and the others 

 secondary asters, following Reinke, who has de- 

 scribed a similar aster formation in the perito- 

 neal cells of the larval salamander. 



Whether the primary asters are formed di- 

 rectly by an actual union and evanescence of 

 several of the secondary asters, I am not pre- 



pared to say at present. Many things indicate 

 that this is the case. At any rate, the two pri- 

 mary asters continue to grow ; a minute dark 

 brown sphere, the centrosome, appears in the 

 center of each, itself surrounded by a lighter 

 brown area, astrosphere, or centrosphe.re, from 

 which the purplish rays diverge in every direc- 

 tion. I am convinced that these two asters 

 and their centrosome are formed by a modifica- 

 tion of cytoplasmic structures. They usually 

 arise at a considerable distance from each other 

 and from the germinal vesicle, and while the 

 nuclear membrane and the nucleolus are still 

 intact. They are the asters and centrosomes of 

 the first maturation spindle. 



After the spindle is formed, it remains for a 

 little while near the germinal vesicle. Each 

 centrosome divides into two ; the nuclear mem- 

 brane disappears ; the chromosomes and the 

 nucleoli are drawn up to the region of the equa- 

 tor of the spindles, and the whole spindle 

 swings around to its definitive position at the 

 periphery of the egg, and perpendicular to the 

 surface. The light brown astrospheres at the 

 poles of the spindle contain each a pair of cen- 

 trosomes. The spindle remains in this condi- 

 tion until the egg is fertilized. 



I have artificially fertilized the eggs of Chas- 

 topterus after they had been in sea water an 

 hour and twenty minutes, yet they developed 

 normally. Immediately after the entrance of 

 the sperm the maturation processes are re- 

 sumed. The first maturation spindle, which 

 has remained up to this time in the equatorial- 

 plate stage, now passes through the succeeding 

 phases of mitosis, which result in the formation 

 of the first polar globule. During these pro- 

 cesses the two centrosomes in the aster at the 

 inner pole of the spindle move apart, and a 

 small central spindle is formed between them. 

 The centrosphere fades away and the rays of 

 protoplasm converge to the two centrosomes. 

 The centrosomes at the outer pole of the spin- 

 dle are carried into the polar globule and there 

 degenerate. A delicate Zwischenkorper is 

 formed at the junction of the polar globule and 

 egg, but it soon vanishes and has nothing to do 

 with the formation of the second maturation 

 spindle. (Compare Mathews, Jour. Morph. 

 X., No. 1, p. 334.) The small spindle before 



