216 MEAD. 
EXPLANATION OF PLATE XVIII. 
Fic. 34. Two sections combined as indicated by the arcs of circles. First 
polar globule lost, second polar globule bent over. Male pronucleus and aster 
with two centrosomes in the centre of the egg. 
Fic. 35. Two polar globules, A.g.1 and p.g.2; Zwischenkorper, z. Cluster of 
vesicles, 9, formed from chromosomes left in the egg after the second matura- 
tion-division. In the midst of the vesicles lies the centrosome towards which the 
rays converge. In the midst of the egg the male pronucleus and aster with one 
of the two centrosomes in sight (the other in the next section). The rays of 
both sperm and egg-asters break up near the periphery into the network. 
Fic. 36. The two male asters with centrosomes and long rays are in defini- 
tive position; sperm-nucleus, ¢, in rather unusual position, z.e., between asters 
and female pronucleus, the vesicles of which are larger than in preceding figure. 
Anomalous case of one vesicle reaching the male pronucleus before the others. 
Aster with rays (two sections combined). 
Fic. 37. Two polar globules, 1 and 2; Zwischenkorper, z. Vesicles of female 
pronucleus very large and close together. Male pronucleus in rather unusual 
position at side of asters. Spindle between the latter. Rays of asters extend to 
the periphery and converge to the centrosomes themselves, as in preceding figure ; 
no centrosphere. 
Fic. 38. Later than Fig. 37. Vesicles of female pronucleus uniting, a few 
faint rays converging to their midst. Male pronucleus and asters in usual 
position. (Two sections combined.) 
Fic. 39. Two polar globules, 1 and 2; Zwischenkorper, z. Male and female 
pronuclei about to unite. Two male centrosomes and asters. Female aster has 
entirely disappeared. 
Fic. 40. Male and female pronuclei have united. The male asters on either 
side. Centrosomes are clear and centrospheres are beginning to form about 
them. The rays diverging from the latter are commencing to break up into a 
reticulum near the periphery. Round nucleoli in the pronuclei. Signs ¢ and Q 
transposed. 
Fic. 41. First cleavage-nucleus elongated; its membrane commencing to 
break down at the ends. The two sperm-asters with centrosomes and centro- 
spheres ; rays comparatively short. Signs ¢ and 9 transposed. 
Fic. 42. Somewhat later stage. Mere traces of nuclear membrane now visi- 
ble. Chromatin, nucleolus, 7.,in middle, centrosomes and centrosphere larger 
than in preceding figure. 
Fic. 43. Cleavage-spindle in metaphase. Chromatin in two clusters, and 
nucleoli scattered among the chromatin loops. Centrosome in right aster elon- 
gated in preparation for a division. Signs g¢ and 9° transposed. 
Fic. 44. Later stage. Centrospheres more highly developed; centrosomes 
dividing in anticipation of next cleavage. 
Fic. 45. Daughter-chromosomes divided and beginning to migrate toward 
the two poles of the spindle, leaving nucleoli in the middle. Daughter-chromo- 
somes moving apart. 
