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bulletin: museum of comparative zoology. 



III. Ontogenetic Processes occurring in Protoplasmic Layers. 



The ontogenetic processes occurring in protoplasmic 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. 



IIP. Of the growth processes affecting area, we may distinguish 

 (a) such as take place in the walls of a hollow sphere or cylinder from 

 ((3) 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 elements 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 recoguize three cases in the 

 differential growth processes occurring in 

 a sac: (a) unequal growth in the different 

 axes ; (b) 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- 

 duced from the spherical ; as in the Sycandra 

 larva (Figure 14) ; in planuhe (K. & H., Figs. 

 14, 30, 32, 51) ; in the larvae of various worms 

 (K. & H., Figs. 144, 158, 159) ; in the Echinoid 

 blastula (K. & H., Figs. 173, 176) ; and in the 

 larvae of some Moll u sea (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 planulse, 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 



Fig. 14. 



Fig. 14. Sycandra embryos. A, 8-cell stage; B, larva. See K. & H., Figs. 2 

 and 3. 



