12 HISTORY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 



jectured by Ryder (No. 4, p. 11). Kupffer 6 takes the same view of their 

 disappearance, while Hoffmann 6 is disposed to think they do not dissolve, 

 but merely become invisible. 



The ripe egg appears to be perfectly spherical, and measures from 

 .85 to .92 mm. in diameter. The formative material forms a distinct 

 cortical layer of nearly uniform thickness. The egg membrane appears 

 to be everywhere in contact with the cortical layer at the time of deposit, 

 but we cannot deny the possibility of the existence of a very small perivi- 

 telline space. Although the cortical layer appears to have no discoidal 

 thickening at any point, it is not improbable that it may be a little thick- 

 ened around the first polar spindle, which, as we shall see, is found near 

 the micropyle only a few seconds after the milt has been added. That this 

 spindle is already formed in the mature unfertilized egg, there is little 

 room to doubt. A few moments after fertilization we find it in a radial 

 position, and at this time, although no proper disc exists, the cortical layer 

 is considerably thicker at the formative than at the opposite pole. 



2. Surface Views of the Blastodisc. — In surface views, the first 

 indication of a polar disc is a shadowy ring (PI. XX. Fig. 3). In one case 

 we saw this ring five minutes after fertilization, and in it was a clear space 

 containing a nuclear body. This body must have been either the spermatic 

 element or the first polar spindle. It was probably the spindle, as the 

 male pronucleus usually requires fifteen or twenty minutes for its develop- 

 ment. The form of the early blastodisc varies considerabky. Sometimes 

 it assumes immediately a conical form, with a rounded summit looking 

 toward the centre of the egg, and then the shadowy ring appears cor- 

 respondingly early ; at other times the centralization is less pronounced, 

 the inner face of the disc being more regularly convex for some time, 

 and then the ring is both later and fainter. As most of the eggs obtained 

 from the sea a few minutes after normal fertilization exhibited a conically 

 thickened disc, it is safe to conclude that this is the usual form. The 

 ring grows steadily larger and more distinct ; and just before the appear- 

 ance of the first cleavage groove it loses its perfectly circular form, and 

 becomes slightly elongated in the direction of one of its diameters. 



3. Optical Sections of the Blastodisc. — Let us now follow these 

 changes in optical sections. Figs. 40-52, Plate XXI., show a series of stages 



6 C. Kupffer. Die Entwickelung des Heriiigs im Ei. Jaliresb. d. Comm. z. wiss. Untersuchung der 

 Deutsehen Meere in Kid, 1874-76. 



fl C. K. Hoffmann. Zur Ontogenie der Knochenfische, Amsterdam, 1SS1, p. 88. 



