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ZOOLOGY: E. G. CONKLIN 
nutrition of the embryo, in the latter for bringing the sperm into union 
with the egg; but in neither case is this differentiated cytoplasm directly 
concerned in heredity. The highly differentiated cytoplasm of the 
spermatozoon is either left outside the egg when its nucleus enters, or 
it undergoes de-differentiation within the egg; at the same time the egg 
cytoplasm ceases to form yolk, while the yolk which has been formed 
is gradually used up in the nourishment of the embryo. Consequently 
since these particular differentiations of the germ cells disappear after 
the union of egg and sperm it has been generally supposed that all 
cytoplasmic differentiations of these cells are wiped out at this time, 
and that the first differentiations of the new individual begin at the 
moment of fertilization in a wholly undifferentiated cytoplasm. 
In the higher animals at least most of the cytoplasmic differentia- 
tions of the spermatozoon are lost after it enters the egg, though some 
differentiations such as centrosome, plastosomes and archiplasm may 
persist; however there is the most positive evidence that many differ- 
entiations of the egg cytoplasm persist and play an important part in 
embryonic differentiation. 
2. Egg Differentiations which persist in Embryo and Adult. — (i) Polarity. 
The polarity of the egg invariably determines the polarity of the em- 
bryo and adult. In all animals the chief axis of the egg becomes the 
chief axis of the gastrula, and this becomes the chief axis of the adult 
in sponges and coelenterates (protaxonia) , or, as in all other metazoa 
(heteraxonia) , this axis is bent on itself by the greater growth of the 
gastrula on its posterior side so that the chief axis of the adult is a modi- 
fication of the gastrular axis. In either case the polarity of the unfer- 
tilized egg determines the localization of developmental processes and 
ultimately the polarity of the developed animal. 
(2) Symmetry. In most animals the egg is spherical in shape and 
appears to be radially symmetrical, nevertheless observation and experi- 
ment show that such eggs are sometimes bilateral, as is probably the 
case in Amphioxus, ascidians, fishes and frogs. In the case of the 
frog's egg it was long believed that the plane of bilateral symmetry was 
determined wholly and exclusively by the path of the spermatozoon 
within the egg; more recently it has been shown by Brachet (1911) that 
primary bilateral symmetry is present before fertilization, though after 
fertilization the plane of symmetry may be shifted into the path of 
the spermatozoon. It is probable that all bilateral animals come from 
eggs which show a similar primary bilaterality and that this differen- 
tiation precedes fertilization. In cephalopods and some insects all 
the axes and poles of the developed animal are already recognizable 
