204 MEAD. [Vou. XIV. 
each other between the poles of the male amphiaster, forming a 
spherical nucleus — the first cleavage-nucleus (Figs. 39, 40).! 
The male asters reach the height of their development as the 
pronuclei come together, and the two centres, connected by a 
spindle, are already widely separated. Each centre contains a 
clearly defined and easily demonstrable centrosome, which is 
soon surrounded by an incipient centrosphere, from which the 
protoplasmic rays extend throughout the whole egg (Figs. 39, 
40). The pronuclei elongate, while the poles of the spindle con- 
tinue to move further apart, and later the nuclear membrane 
gradually disappears. An actual fusion of the pronuclei does 
not take place (Figs. 40, 41), and, even after the membrane has 
vanished, the chromosomes derived from the egg and from the 
spermatozoon respectively are often seen to be in separate 
groups (Fig. 43). As the cleavage-nucleus begins to elongate, 
the rays of the amphiaster become less extensive, breaking up 
into a network in the outer portion of the cytoplasm, and when 
the nuclear membrane disappears, centrospheres become more 
strongly developed around the centrosomes. The chromosomes 
arrange themselves in an equatorial plate, and the metaphase 
of the first cleavage-amphiaster is established (Figs. 43, 44). 
The centrosomes at the poles of the spindle by this time 
have each divided in anticipation of the next mitosis, and the 
centrospheres have increased in size. Several nucleoli lie scat- 
tered among the chromosomes in the equatorial plate (Fig. 45). 
Like the chromosomes they stain with haematoxylin, but are 
easily distinguished by their irregular shape and arrangement 
(Fig. 46). 
The chromosomes split longitudinally according to the hetero- 
typical method (Figs. 44, 45), and the daughter-chromosomes 
recede toward the opposite poles (Fig. 46). In the cross- 
section of the spindle I have often counted the chromosomes, 
and found them always to be eighteen in number (Fig. 47), as 
would be expected, since there were nine in the maturation- 
spindle (Fig. 18). When the daughter-chromosomes have sepa- 
rated to some distance, the nucleoli are seen in their original 
position midway between the poles of the spindle (Figs. 46, 
1 The signs ¢ and ? in Figs. 40, 41, and 43 should be transposed. 
