HISTORY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 35 



The periplast, beginning as a minute clear area, rapidly increases in 

 size; and in the more advanced phases of nuclear division it becomes 

 voluminous, and very conspicuous owing to its non-stainable properties. 

 The appearance of the periplast in various stages of development is shown 

 in Plates XXIV. to XXXI. There is little to be said about its structure 

 and composition. No centrosome is recognizable in Plate XXIV., but in 

 Plate XXX. Figure 2, we see at the centre of the clear periplast an area 

 with irregular outlines, towards which the astral rays converge. Is this the 

 centrosome, or is it a new periplast forming within the old ? Vejdovsky's 

 observations would favor the latter view, but we will not venture, on the 

 evidence at hand, to express an opinion. In figure 1, Plate XXVII., this 

 doubtful body has a stellate form. 



The cytoplasm around the periplasts is denser than elsewhere in the 

 blastodisc, and is deeply stained with osmic acid, sometimes presenting the 

 picture of two dark rings with a connecting axis, in which lies the spin- 

 dle (Fig. 6, PI. XXIV.). Figure 7 represents a median section of the disc 

 shown in Figure 6. 



2. The Division of the Nucleus. — Let us now turn to the history 

 of the nucleus, and briefly notice the changes it undergoes during the 

 development and separation of the polar asters. There are two elements 

 constantly in view during these changes ; namely, the chromatic elements and 

 the nuclear fluid. We shall have little to say about the spindle fibres and 

 the so-called connecting fibres between the nuclear plates. Van Beneden, 

 Boveri, and Rabl maintain that these are two distinct sets of fibres, and 

 we do not question the accuracy of their conclusion on this point, although 

 we are not able to make out such a distinction in the fish egg. 



When the asters first appear, the nucleus has the oval form shown in 

 Plate XXIII., and the chromatic elements have not yet assumed the char- 

 acteristic form of granules. The concentration leading to this form is in 

 progress, however, and five minutes later (forty-five minutes after fertiliza- 

 tion) the equatorial plate becomes very distinct (Fig. 1, PI. XXIV.). The 

 asters have enlarged a little, and moved from the poles of the nucleus to a 

 distance nearly equal to their diameters. The nucleus has approximately 

 the same volume, but has assumed the barrel-shape so characteristic of this 

 stage. The poles of the nucleus are now flat, and its sides evenly convex. 

 The nuclear fluid has not escaped, and we believe thai it undergoes morphological 

 division. Fixation with osmic acid enables us to affirm this with the utmost 

 positiveness. 



