PRATT: IMAGINAL DISCS. 
253 
a muscid, in which, according to Graber (’ 89 ) , the hinder portion of 
the egg is provided with it, while the forward portion lacks it. 
Graber does not find even the thinnest peripheral protoplasmic 
layer in the forward half of the egg of this fiy, but asserts that the 
yolk-balls abut immediately on the vitelline membrane, — an obser¬ 
vation the accuracy of which seems to me on general principles 
extremely doubtful. 
Melophagus, however, as will be seen, acquires a Keimhautblas- 
tem before the completion of the blastoderm. 
The processes leading to the maturation and fertilization of the 
egg were not observed. The segmentation-nuclei were first ob¬ 
served forming an irregular group near the centre of the egg. 
Figure 1 (PI. 1) represents a section of an egg in which about ten 
of these nuclei were counted, all of which were migrating through 
the yolk towards the periphery. Around each nucleus is a 
zone of clear protoplasm, possibly the result of the absorption of 
yolk granules near it. Each nucleus, moreover, as it moves toward 
the surface, leaves behind it a path of clear protoplasm. Only four 
of these nuclei are seen in the section, but portions of the paths of 
others are visible. It is necessary to call attention to the irregular 
nature of the migration of these nuclei. In Musca the earlier seg¬ 
mentation-nuclei are described by Blochmann and Voeltzkow (’ 89 ) 
as advancing in very regular order towards the surface of the egg 
from the centre and arranging themselves in positions approxi¬ 
mately parallel to the surface as they advance. Kowalevsky (’86) 
observed that the segmentation-nuclei in Musca arrive at the sur¬ 
face of the egg first in the hinder portion, then in the forward, 
and lastly in the central portion. Voeltzkow (’ 89 ) observed in the 
same animal that at all portions of the egg they arrive at the-same 
time. In Melophagus, as will be seen in Figure 1, most of the 
nuclei are near the centre of the egg, but are advancing quite irreg¬ 
ularly towards the surface, as is indicated by their plasma-paths. 
The peripheral plasma-layer in this egg is very thin, being but 5 /z 
thick. 
Figure 2 shows a section of an egg in which blastoderm formation 
has advanced much farther. The paths of the advancing nuclei 
form here a network among the yolk oranules. Some of the nuclei 
are seen to be in the process of division ; but the great majority of 
them are spherical bodies with distinct nuclear membranes and few 
chromatin granules. Many have reached the periphery of the egg. 
