ZOOLOGY: E. G. CONKLIN 
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in the egg before fertilization. Symmetry, therefore, as well as polarity 
is derived from the egg and not from the sperm. 
(J) Inverse Symmetry {Asymmetry) . In many animals the right and 
left sides of the body are not completely alike, and this is especially 
true of internal organs. This asymmetry is especially well developed 
in gasteropods in which certain organs of one side of the body are entirely 
lacking; some species or individuals have these asymmetrical organs 
on one side, others on the other side, and correspondingly the snail shell 
coils in a clock- wise direction in one case, an anticlock-wise direction 
in the other. It was discovered by Cramp tom (1894) and Kofoid (1894) 
that in sinistral species the direction of certain cleavages of the egg 
{viz, the third to the sixth) was the reverse of the corresponding cleav- 
ages in dextral species and Conklin (1903) showed that the first and 
second cleavages also were in opposite directions in dextral and sinistral 
snails. Consequently the 'inverse symmetry' of these snails may be 
traced back through the later and earlier cleavage stages to the unseg- 
mented egg itself which is inversely symmetrical in sinistral as com- 
pared with dextral forms. 
(4) Types of Egg Organization. The polar differentiation of an egg 
is manifested particularly in the localization of different kinds of ma- 
terials in different parts of the egg. These materials may be inert 
pigment or yolk, but their localization by the activity of the cytoplasm 
indicates a definite pattern of organization in the cytoplasm. This 
pattern of egg cytoplasm differs greatly in certain phyla, there being 
a coelenterate type, an echinoderm type, an annelid-mollusk type, 
and a chordate type. The type of egg organization foreshadows the 
type of adult organization; in ascidians for instance distinct cytoplasmic 
substances are found in the egg in the same relative positions and pro- 
portions as the ectoderm, endoderm, mesoderm, notochord and nerv- 
ous system of the embryo. 
That the fundamental pattern of egg cytoplasm is not influenced by 
the spermatozoon is proved by the following facts: 
a. It exists before fertilization, or it appears so soon after that it 
could not have been caused by the sperm. 
h. In heterogeneous fertilization the pattern of the egg is not changed 
by the foreign sperm. 
c. Natural or artificial parthenogenesis demonstrates that this pat- 
tern exists in the absence of fertilization. 
These as well as other facts such as the correspondence between 
the size of the egg and the size of the embryo (Morgan) ; the transmis- 
sion of chromatophores and peculiarities of leaf coloration by the fe- 
