CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 221 
stead of a single attraction sphere, there are two, both well defined and 
at a considerable distance apart. An examination of other eggs of the 
same lot shows how the condition here existing has come about. The 
male archoplasm moving in advance of its pronucleus (cf. Fig. 5) has 
gradually eiongated transversely to its line of progress, arranged itself 
about two centres instead of one, and finally constricted itself into two 
distinct. spherical masses, which move apart, and by their combined 
action on the male pronucleus draw it forward to a position midway 
between them, so that its long axis lies in the line joining their centres. 
The female pronucleus (Fig. 8) has approached to within a short dis- 
tanco (about one fifth the diameter of the egg) of the male pronucleus. 
It has grown to an equal size with the male pronucleus, and, like 
it, contains large chromatic granules. No trace of an archoplasmic 
body can be seen in comnection with it, nor in either of tho adjacent 
sections. However, what are unmistakably the remains of one are visi- 
ble three sections behind the female pronucleus, (See Fig. 9; compare 
also Figs. 11 and 12.) This archoplasmic body shows signs of disinte- 
gration, being rather diffuse and exerting apparently no attractive 
influence on the egg protoplasm. The female pronucleus has clearly 
passed beyond its control, and is now advancing rapidly to unite with 
the male pronucleus. One might doubt that the body deseribed is 
identical with a female archoplasm, were it not perfectly constant in its 
appearance at this stage behind the female pronucleus in the path of the 
latter from the point where the polar globules were formed toward the male 
pronucleus. Moreover, though diligent search has been made, a similar 
body has never been found at this stage in any other portion of the 
ovum, except in conneotion with the male pronucleus. 
In from five to ten minutes after the stage just described the two pro- 
nuclei are seen to have come together (Plate IIT. Fig. 13, and Plate I. 
Fig. 6). They are indistinguishable from each other so far as size and 
optical appearance are concerned, and are flattened against each other, 
but their nuclear membranes remain intact, and there is no mingling of 
their substance until the first cleavage is about to take place. (See 
Plate III. Fig. 14.) At the stage shown in Plate IT. Fig. 7, we saw that 
the male pronueleus was already elongated between its two attraction 
spheres. The female pronucleus is seen in Figure 13 (Plate III.) to have 
joined it while it is still in that condition. Both have further increased 
in size. Very soon the nuclear membranes disappear, the attraction 
spheres move farther apart (cf. Figs. 13 and 14, Plate IIT.), and a spindle 
forms between them, on whose equator are seen the chromosomes. 
