36 HISTORY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 



The phase just described is the nuclear spindle with truncated poles. 

 It is made up of the equatorial plate, the homogeneous fluid or nucleo- 

 plasm, which is strongly darkened with osmic acid, and indistinct spindle 

 fibres. The demonstration of the nucleoplasm is an important point, and 

 we would especially emphasize the fact that in certain other methods of 

 treatment we see nothing of this substance, while the spindle fibres are 

 strongly differentiated. This is the case when Perenyi's fluid is used 

 (Fig. 4, PI. XXVIII.), and sometimes also in our usual method, when the 

 osmic acid has for some reason failed to act with sufficient intensity 

 (Fig. 1. PI. XXX.). 



Although the truncated form of the spindle is the one we usually get, 

 the oval form of the nucleus is sometimes maintained even after the division 

 of the nuclear plate (Figs. 3, 4, PI. XXIX., and Figs. 6, 7, PI. XXI V.). 



Figure 2, Plate XXI Y. (45 minutes) represents a median vertical section 

 of the blastodisc, with the usual form of the spindle, in which the division of 

 the chromatic elements into two plates is taking place; and Figure 3 (50 

 minutes) shows the division completed. Between the two plates we see a 

 clear space, showing that the nucleoplasm has divided simultaneously with 

 the nuclear plate. The astral fields are still more enlarged, and farther 

 apart (cf. Figs. 3 and 4. PI. XXIX.). 



Figures 4, 0, and 7, Plate XXIV., represent more advanced phases of 

 division (50 minutes after fertilization). Figure 4 is from a preparation that 

 shows the daughter nuclei clearly outlined. The nucleoplasm is perfectly 

 homogeneous, with no membrane, yet quite clearly outlined. The chromatic 

 elements still preserve the plate arrangement, and mark the equators of 

 the nuclear spheres. In Figures 6 and 7 the two nuclei have a hemi- 

 spherical form, the median equatorial faces being bounded by the chro- 

 matic plates. Between these plates stretch fine connecting fibres. The 

 phase seen in Figure 4 immediately precedes the condition in Figure 1. 

 Plate XXV., when the cleavage line appears in the blastodisc. The chro- 

 matic elements are now passing from the plate form into that of irregular 

 groups, such as are shown in Figure 2, Plate XXX. 



The nucleoplasm is still recognizable, at least in some of our prepara- 

 tions (Fig. 1, PI. XXVI.). The two nuclei move towards their respective 

 astral centres, and reach the edge of these about the time the elongation 

 of the periplasts begins, preparatory to another division. 



