lO 



ANDREWS. 



often a minute alveolation, which is most unstable as to size 

 and arrangement, instead of the much elongated meshes of 

 Biitschli's structure, which sometimes take its place. 



Although an optically linear, hexagonal, or stretched reticu- 

 lum of Biitschli can be seen in many cases, there are others, 

 and not a few, in which this state of things is preceded by the 

 alveoli being separated from each other by a considerable amount 

 of interalveolar foam; and in place of yielding a true "network" 

 they have a very irregular or subspherical contour. They pre- 

 sent, in short, just that difference of appearance which is seen 

 when the bubbles of an ordinary soap-foam are surrounded by 

 a finer froth and are not in direct lamellar contact with other 

 bubbles of their own size. 



The local tensions in the fine foam are constantly making 

 the outline of the larger bubbles flatten on this side or on that, 

 or on several sides at once. The whole seeming of such proto- 

 plasmic areas is that of an emulsion rather than that of a true 

 foam ; but seeing that the interalveolar stuff is itself a foam, 

 and convertible, and converted into the coarser and more regu- 

 lar structure of Biitschli, has led me to call the whole compound 

 a true foam of varied coarseness. 



Fig. I is a simple, outline diagram of such an area, taken 

 from nature, of the protoplasm of a 

 portion of a starfish egg, before the 

 structure of Biitchli has been more 

 perfected. The smaller vesicles show 

 the finer foam of the ectosarc-like 

 layer, which first forms about the pe- 

 riphery of the mass. In this, also, the 

 network is broader and more irregular 

 than it will be later ; and the same 

 general characters of the internal area 

 are repeated. 



The "granules" in such areas are 

 not by any means wholly confined to 

 the nodes of the network but are found, as drawn, scattered 

 through the finer foam. 



In the endosarc of Vorticellidae, and in their cuticular struc- 



FlG. 



