184 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY 
III. ONTOGENETIO PROCESSES OCCURRING IN PROTOPLASMIO LAYERS. 
The ontogenetic processes occurring in protoplasmio layers may con- 
cern one layer only, or'two or more layers acting in relation to each 
other. 
The processes which take place in one layer are principally growth 
processes affecting on the one hand the area, on the other the thickness 
of the layer. 
III*. Of the growth processes affecting area, we may distinguish 
(a) such as take place in the walls of a hollow sphere or cylinder from 
(B) such as take place on a plane or warped surface. 
a. The areal growth occurring in a hollow sphere or cylinder (sac) 
may be equal in all the axes or elenients of the wall, leading merely to a 
change of size of the sac; or it may be unequal in the various axes or 
elements, producing a change in form. It is 
this latter group which especially interests 
us, and it will therefore be further analyzed. 
1. We may recognize three cases in the 
differential growth processes occurring in 
a sac: (a) unequal growth in the different 
axes; (0) unequal growth at the poles of the 
axis; (c) unequal growth in the various me- 
ridians of the sac. 
These three processes deserve illustration. 
a. By the process of excessive growth in a 
certain axis we have the ellipsoidal form pro- 
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duced from the spherical ; as in the Sycandra 
larva (Figure 14) ; in planule (K. & H., Figs. 
14, 30, 32, 51) ; in the larv of various worms 
(K. & H., Figs. 144, 158, 159); in the Echinoid 
blastula (K. & H., Figs. 173, 176) ; and in the 
larvee of some Mollusca (Figs. 542, 576, 593, 
596) and Bryozoa (Fig. 702), 
b. Through unequal growth at the two poles of (e. g. the chief) axis, 
we have produced such ovoidal forms as planule, in which not only is 
the chief axis elongate, but one pole has grown more than the other, 
(Figure 15.) By unequal growth at the poles are produced also such 
= 
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dy 
Fig. 14. 
Fig. 14. Sycandra embryos. A, 8-cell stage; B, larva. See K. & H., Figs. 2 
and 3. 
