234 



SCIENCE. 



[N. S. Vol. V. Ko. 110. 



mentioned becomes the second maturation 

 spindle by means of which the second polar 

 globule is extruded. 



The two centrosomes, which, as we have 

 seen, are identical with those in the inner aster 

 of the first maturation spindle, move still fur- 

 ther apart and, with spindles between them, 

 take a position on either side of the group of 

 chromosomes left in the egg after the first divi- 

 sion. Tho chromosomes are arranged in the 

 equatorial plate, and the whole spindle swings 

 around to an approximately vertical position 

 directly under the first polar globule. The 

 radiations from both ends of this spindle are 

 long and conspicuous. The centrosome at the 

 inner end usually, perhaps always, divides into 

 two. It is surrounded by a small centrosphere. 



The second polar globule is formed by the 

 usual process of mitosis. In the second polar 

 globule, as in the first, there can be seen for a 

 time the degenerating centrosome of the outer 

 pole of the spindle. A Zwischenkorper is 

 formed, consisting at first of a circle of small 

 dots, like the middle plate of plants, with 

 rays extending in both directions. Later it 

 becomes constricted to a single center with 

 diverging rays, and in this condition it is distin- 

 guishable until the pronuclei have nearly united. 

 The chromosomes left in the egg at the inner 

 end of the spindle group themselves so as 

 nearly to surround the centrosome and its 

 astrosphere. The rays of the latter are numer- 

 ous and long, extending through half the diame- 

 ter of the egg. 



The nine chromosomes swell up into as many 

 vesicles which migrate toward the middle of 

 the egg, and as they do so unite to form one large 

 female pronucleus. The aster and centrosome 

 are carried along with the vesicles for a cer- 

 tain distance, but degenerate before the vesi- 

 cles have united. The rays of this aster, which 

 were very strongly developed when the vesicles 

 were first formed, become weaker and weaker, 

 and finally disappear entirely. While they 

 are still present they converge to the center of 

 the group of vesicles and indicate the position 

 of the centrosome. 



While the processes of maturation have been 

 going on, the entrance of the sperm has wrought 

 profound changes in other parts of the egg. 



The sperm may apparently enter the egg any- 

 where. Soon after it has entered we find near 

 it a minute aster with two centrosomes close 

 together. They lie in a minute astrosphere 

 from which a few short rays diverge. 



These two centrosomes are known as the 

 'male centrosomes,' though in Chsstopterus I 

 am not sure that they are actually carried in by 

 the spermatozoon. A strong presumption is 

 created in favor of this view by the fact that 

 the sperm has in the middle piece two bodies re- 

 sembling centrosomes. However this may be, 

 the male centrosomes separate as the head of 

 the spermatozoon enlarges, to form the male 

 pronucleus, and as they separate, the rays di- 

 verging from them become more and more 

 extensive. 



Besides moving apart, the centrosomes mi- 

 grate toward the center of the egg, the male 

 pronucleus accompanying them, sometimes on 

 one side and sometimes on another, but always 

 near at hand. They finally take a definite po- 

 sition a little to one side of the center of the egg 

 and toward the polar globules. The central 

 spindle which has developed between them lies 

 at right angles to the egg axis. At this time 

 nearly all the cytoplasm of the egg appears in 

 the form of varicose fibrils, radiating directly 

 from the two male centrosomes. 



These enormous male asters are the poles of 

 the first cleavage spindle. They are already 

 connected by a central spindle. The pronuclei 

 come together between the poles and elongate 

 slightly. The nuclear membrane disappears ; 

 the chromosomes arrange themselves in the 

 equatorial plate usually in two distinct groups, 

 and we have the first cleavage spindle in the 

 metaphase. 



During the formation of the cleavage spindle 

 a centrosphere develops about each centrosome, 

 and the rays become very much shorter, for 

 their distal portions break up to form again a 

 cytoreticulum. 



While the chromosomes are undergoing the 

 longitudinal splitting, each centrosome divides 

 into two in anticipation of the next cleavage. 

 The two daughter centrosomes move apart in 

 each astrosphere without disturbing the spher- 

 ical shape of the latter, until the beginning of 

 the telaphase, when the chromosomes at either 



