1896. NEW YORK ACADEMY OF SCIENCES. 169 
i 
tates, as Wilson and Mathews (10) have shown to be the case 
among the Echinoderms. The process extends through the 
surrounding clear region, to the margin of the cytoplasm, where 
it is seen to be crowned by a minute aster with a focal centro- 
some similar in all respects to the egg centrosome in its earliest 
condition, the rays converging directly to the centrosome with- 
out any visible surrounding archoplasmic mass. A division of 
the centrosome which soon follows leads to the formation of a 
minute amphiaster still some distance in advance of the main 
portion of the spermhead. (Fig. 8.) By this time the process 
has disappeared. A “central spindle” connecting the daughter 
centrosomes is not demonstrable in all cases. In cases where it 
appears to be present, it is impossible to decide, owing to its 
extreme minuteness, whether it is a true central spindle or 
merely an accidental meeting of astral rays. In later stages it 
entirely disappears. 
While the second polar anaphase is in progress both the 
sperm amphiaster and the sperm pronucleus increase in size and 
the latter assumes a vesicular structure and approaches the am- 
phiaster. By the time the female vesicles are formed and aggre- 
gated, and their radiations are disappearing, we find the male 
pronucleus assuming a hemispherical shape with the base closely 
applied to the amphiaster, one centrosome and aster at each 
pole. (Fig. 10.) As yet no centrosphere has appeared, the rays 
still converging directly to the centrosomes. 
Considerable variation exists in regard to time of fusion of 
the vesicles. In some cases they fuse extremely early, in others 
they reach the male pronucleus and fuse with it while still 
separate though closely aggregated. This appears to be related 
to the position of the male pronucleus at the time of the second po- 
lar anaphase. Ifthe former is still some distance away the vesi- 
cles may have time to fuse before meeting it, but if, on the other 
hand, the sperm nucleus has already advanced so as to lie near 
the vesicles when the latter are formed they may not have time 
to fuse before copulation of the pronuclei. The female pronu- 
clei or group of vesicles now applies itself to the base of the 
male pronucleus between the asters,and in all cases so far ob- 
served, fuses with it completely. giving rise to a segmentation 
nucleus in which there is no visible distinction between paternal 
and maternal chromatin. After,or even before, the wall separa- 
ting the two pronuclei has faded, the chromatin is observed to 
have thickened at various points to form the commencement of 
the chromosomes. This fact renders it highly probable that 
there is no actual fusion of the chromatin, but that each half 
retains its individuality and that one-half of the resulting chro- 
mosomes are of paternal origin and the other half of maternal, 
