450 
EDWIN G. CONKLIN 
DESCRIPTION OF FIGURES 
All figures are those of the living eggs or embryos of fresh-water pulmonates, 
and most of them are freehand drawings. 
1-22 Normal development of the eggs of Physa and Planorbis, showing the 
normal distribution of the yellow and clear substances during the maturation, 
fertilization and early cleavages of the egg. The yellow substance is ultimately 
confined very largely to the entomeres and mesomeres at the vegetal pole. 
23-26 Successive stages in the development of an egg of Lymnaea centrifuged 
in the germinal vesicle stage. The nucleus lies in the clear zone and in this zone, 
the first maturation spindle forms. During the formation of the first polar body, 
which always lies on the clear zone, the egg constricts in the region of this zone, 
but this constriction never leads to complete division. 
27-30 Successive stages in the development of an egg of Lymnaea centrifuged 
during the second maturation division. A deep constriction appears through the 
clear zone, which often leads, as in this case, to the complete separation of the gray 
sphere. This gray sphere is really the second polar body to which the normal first 
polar body is attached, and it undergoes no further development. The yellow 
part of the egg gives rise to a normal embryo and adult. 
31-33 Successive stages in the development of an egg of Physa centrifuged 
22 min. after the maturation divisions. The eggs were drawn out into cylinders 
(fig. 31), and in many cases the first cleavage was unequal, the small cell being 
yellow and the large one clear. Such eggs are capable of normal development. 
34-37 Successive stages in the development of an egg of Lymnaea centrifuged 
after maturation, in which one of the first two blastomeres is chiefly gray, the other 
chiefly yellow. In the rotating embryo (fig. 37) the gray substance is largely 
located in the anterior half, the yellow in the posterior half. 
38-41 Stages in the development of an egg of Lymnaea centrifuged after 
maturation, in which the first two blastomeres have an equal portion of the 
three substances, the axis of stratification being in the plane of the first cleavage. 
In the rotating embryo (fig. 41) the yellow substance is largely located in the 
right half of the body, the gray in the-left. 
42-45 Stages in the development of an egg of Lymnaea in which the axis of 
stratification is oblique to the first cleavage plane, and in which the distribution 
of substances in the embryo (fig. 45) is oblique to the chief longitudinal and cross 
axes. 
46, 47 Young snails of the genus Physa, which have developed from eggs simi- 
lar to those shown in figs. 31-33. In fig. 46 there is a partial inversion of symmetry, 
the apex of the shell coiling forward and to the right, whereas in normal cases it 
coils backward and to the left. The heart also is transposed in position. In fig. 
47 the young snail is normal, except that the whole buccal region, together with the 
radula is everted. This is a rather common abnormality in these centrifuged eggs. 
