castle: embryology of ciona intestinalis. 219 



neut than those ruuuiug in other directions. Along them as radii 

 doubtless protoplasm is passing to augment the yolk-free area. Nothing 

 in the nature of " fibres " has been observed in them. Excentrically 

 situated in the yolk-free area (Fig. 4) is seen the male pronucleus, a 

 perfectly clear oval body, with a delicate but sharp boundary. Its long 

 axis lies radially with reference to the attraction sphere, which mani- 

 festly exerts on it a directive influence. Figure 4 represents the eleventh 

 of a series of eighteen sections. The sixteenth section of the series is 

 shown in Figure 3. It contains the second maturation spindle, at either 

 end of which is an attraction sphere in the centre of a slight accumula- 

 tion of protoplasm. The chromosomes cannot be clearly made out, but 

 perhaps lie aggregated in a small dark mass close down against the 

 attraction spheres. It is evident that the amount of chromatin in- 

 volved in this division is less than in the case of the first maturation 

 division (cf. Fig. 1). The obliquity of the plane of sectioning to the 

 dorso-ventral axis of the egg makes this spiudle appear to lie quite a 

 little below the surface of the egg. Such, however, is not the case ; it 

 comes close up to the surface, but obliquely, not vei'tically, as did the 

 first maturation spindle. Indeed, an examination of other specimens, 

 less advanced, shows that it first appears in a horizontal position, i. e. 

 at right angles to the direction of the first maturation spindle as seen in 

 Figure 1, but later rotates so that one end of the spindle lies deeper in 

 the egg than the other.^ The first polar globule does not really lie in 

 this section, but has been projected there from its real position oh the 

 margin of the next section, the seventeenth of the series. 



In Figure 5 is represented a section, the fifth of a series of sixteen, 

 through an egg of Series A, lot 3, killed twenty minutes later than 

 lot 1 of the same series (cf. Fig. 1), The section passes obliquely in a 

 dorso-ventral direction, unlike those shown in Figures 2-4, which were 

 more nearly horizontal. On the ventral margin of the section is seen the 

 cap of protoplasm which as early at least as the beginning of maturation 

 covei-ed that side of the egg. The male archoplasm has moved deeper 

 into the egg, and its attractive influence has been extended so that it is 

 now manifested over the greater portion of the section. In consequence 

 of this attraction on the protoplasm the area free from yolk has con- 



1 A rotation of the maturation spindles from an original tangential to a radial 

 position has been observed repeatedly in other animals ; in the case of the second 

 spindle, the tangential position is doubtless correlated with the derivation of its two 

 archoplasmic masses from the single arehoplasmic mass left in the egg after the 

 completion of the first maturation division. 



