PRATT: IMAGINAL DISCS. 
255 
The nuclear paths in this egg have entirely disappeared. Their 
disappearance undoubtedly accounts largely for the increase in the 
thickness of the blastema. 
The steps in the formation of the blastoderm between this stage 
and the completed blastoderm I have not observed, but I believe 
the concluding process to be as follows : The nuclei and their pro¬ 
tuberances still further increase in number, and the former decrease 
in size; the latter are brought into immediate contact with one an¬ 
other and their walls fuse and become the lateral boundaries of the 
future blastoderm cells; these boundaries are carried still farther 
toward the inner surface of the blastema, and the c£lls are finally 
completed by the formation of a wall bounding their inner 
ends. In the completed blastoderm (PI. 1, Fig. 7) there is still a 
narrow blastema present, and the diameter of the nuclei has fallen 
to 2.5 fx. No inner or secondary blastema, such as is described in 
Musca by Graber (' 89 ) and other authors, is present. I did not 
observe the formation of the pole-cells. 
2. The Formation of the Mesoderm and of the Proctodeum and 
iStomodeum. — At the time of its completion, the blastoderm is 
composed of narrow cells of equal height throughout. In the 
centre of the egg are numbers of so-called yolk-nuclei. Inasmuch 
as these yolk-nuclei are homologous to the primitive endoderm in the 
gastrulation of the majority of animals, as has been determined by 
Heymons (’ 95 , ’ 97 ) and other authors, the stage of development in 
the ontogeny of Melophagus in which the blastoderm is completed, 
as represented by Figure 7, would be the gastrula-stage. The next 
step in the development is the formation of the germinal plate and 
the median mesodermal band. The cells on the concave (dorsal) 
side of the egg diminish somewhat in height (PI. 2, Fig. 8); those 
on the convex (ventral) side rapidly proliferate along the median 
line; a slight depression appears in the blastoderm along the mid- 
ventral line, on the inner surface of which a ridge of cells, the 
primitive mesoderm ( crs . ms'drm .), is raised, projecting into the 
yolk. The ectodermic cells of this ventral region become elongated, 
and with the mesodermic cells constitute the germinal plate. The 
plate does not, however, confine itself to this portion of the egg, but, 
as is common in the Diptera, quickly extends itself to portions of the 
dorsal side, the thickened ectodermic portion encircling both poles 
of the egg and occupying about a third of the dorsal surface at each 
end (PI. 2, Fig. 9). The primitive mesodermal plate also extends 
