320. M. D. HILL. 
thread, and then is split into eight chromosomes. The details 
I have not been able to follow (fig. 9). 
Formation of the first polar body.—The polar bodies 
are given off shortly after the ova are shed into the sea water, 
irrespective of whether they are fertilised or not. The 
nucleus loses its membrane, a spindle is formed, and the 
eight chromosomes are arranged in the equatorial plane. As 
to what follows I do not wish to lay down any absolute facts. 
The polar spindles are so excessively small, and the chromo- 
somes lie so close to one another, that accurate observation is 
a matter of extreme difficulty. From the study of a large 
series of sections, however, I am convinced that the réle played by 
the chromosomes is very different from what has been described 
for Ascaris, and on which so many theoretical speculations 
have been based. The eight chromosomes of the first polar 
spindle lengthen out, become dumbbell-shaped, and finally 
divide in the middle (fig. 10). Eight chromosomes pass into 
the first polar body, which also divides karyokinetically into 
two, each having eight chromosomes. (I have never counted 
more than six or seven chromosomes in the products of 
division of the first polar body, but I think it may be taken for 
granted that there must be eight [fig. 11].) The chromo- 
somes left in the first polar spindle again divide in the same 
manner (?), and about eight—certainly more than four—chro- 
mosomes pass into the second polar body (I have counted six 
distinctly), and eight (?) remain. To exactly determine the 
number of chromosomes left in the female pronucleus after 
the formation of the second polar body, and before it passes 
into the resting condition, is a matter of great difficulty, as the 
time between the two phases is very short (fig. 12). I have 
counted four, six, seven, eight, and nine in different instances. 
This discrepancy is partly due to the great tendency the 
chromosomes have to lump themselves together into one mass, 
so that the female pronucleus resembles the nucleus of the 
ovogone in having a large “nucleolus” of chromatin. This 
is broken up into a network as soon as the nucleus develops 
a membrane and passes into the resting condition. It then 
