CASTLE : EMBKYOLOGY OF CIONA INTESTINALIS. 221 



stead of a siugie 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 elongated 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 appi'oached to within a short dis- 

 tance (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 connection with it, nor in either of the 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- 

 gi-ation, 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 described is 

 identical with a female archoplasm, were it not perfectly constant in its 

 appearance at this stage behind the female jji'omtcleus in the path of the 

 latter from the jjoint where the polar globules were formed toivard 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 connection 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 III. Fig. 13, and Plate I. 

 Fig. G). 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 II. Fig. 7, we saw that 

 the male pronucleus 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 

 jn size. Very soon the nuclear membranes disappear, the attraction 

 spheres move fiirther apart (cf. Figs. 13 and 14, Plate III.), and a spindle 

 forms between them, on whose equator are seen the chromosomes. 



