630 Gro. W. TANNREUTHER, 
divisions ‘are represented in Fig. 57. We can readily distinguish 
the eight nuclei that are situated more posterior from those that 
are found more anterior. This interesting phenomenon has no special 
bearing on the later development except that the posterior half of 
the blastoderm approximately is formed from the offspring of one 
daughter nucleus, while the anterior half is formed from the offspring 
of the other daughter nucleus. The final outcome is, since only a 
very small part of the posterior end of the blastoderm forms the 
germ band, that the cleavage nuclei resulting from one of the 
daughter nuclei, that is situated more posteriorly give rise to the 
entire germ band, part of the serosa and part of the yolk nuclei; 
while the offspring from the more anterior daughter nucleus gives 
rise only to a part of the serosa and part of the yolk nuclei. 
The formation of the blastoderm begins uniformly over the 
entire surface of the egg, except at the extreme posterior end which 
remains almost entirely free from cleavage nuclei while the blastoderm 
is forming (Fig. 58). The cleavage nuclei pass to the extreme 
surface of the egg (Fig. 58) and enter the peripheral layer of 
cytoplasm. The cleavage nuclei that do not pass to the periphery 
of the egg become active centers for the digestion of the yolk 
spheres. WHEELER (1889) found in Platta that all the cleavage 
nuclei passed to the surface of the egg and took part in the 
formation of the blastoderm, while the center of the egg was devoid 
of nuclei and that the yolk cells or nuclei appeared later by the 
wandering of cells from the blastoderm into the interior of the egg. 
The point at which cleavage nuclei reach the surface of the 
egg varies in different groups of insects. In Muscidae (GRABER) the 
formation of the blastoderm begins at the posterior end of the egg. 
In Blatta (WHEELER) the first cells that form the blastoderm appear 
on the ventral surface, i. e., the future ventral surface of the embryo. 
In Pieris (BopretzKy) the blastoderm cells first appear at the anterior 
end or pole of the egg. In Aydrophilus (HEmER) the blastoderm 
first forms around the middle of the egg as a transverse girdle 
somewhat nearer the posterior pole of the egg and develops last 
at the poles of the egg. 
In M. salicis the blastoderm begins uniformly over the entire 
surface of the egg except at the posterior pole. The cleavage nuclei 
n the formation of the blastoderm divide but once after reaching 
the surface of the egg (Fig. 59). 
The blastoderm before invagination never becomes more than 
+ paf ae. we al 
