vol.i.] Ritter-Davis. — Enteropneusta. 177 



a conservative estimate for a typical full grown larva. When 

 now it is recognized that the blastocoel contains no organs 

 excepting the mesoblastic pouches, and that these occupy but 

 very little space, the two posterior pairs being wholly collapsed 

 and thin walled; and further, that the great part of the bulk 

 comes into existence in the course of the development of the larva, 

 and disappears again in large part as metamorphosis progresses, 

 it becomes quite clear that it must be a positive element in the 

 organization and life career of the larva. Its significance is 

 pretty certainly as a container of the mass of secreted material 

 by which the diminished specific gravity of the larva is brought 

 about. What is the source of this mass? 



The sparceness of free cells within the mass is conclusive 

 evidence that it cannot be the product of these cells. In 

 the sections of some specimens scarcely any such cells at all are 

 recoguizable while in others they are somewhat more abundant. 

 Nowhere, however, are they numerous. We fail, likewise, to 

 find indications of a secretory activity in any part of the ecto- 

 derm that might be the source of the mass, and the mesoblastic 

 pouches show even more clearly than does the ectoderm that they 

 cannot produce it. The walls of these structures remain 

 extremely thin during all the larval period. These three possible 

 sources being excluded there remains only the digestive tract 

 to look to; and here we do not look in vain. The stomach /rail 

 almost certainly produces the muss. Spengel has called attention 

 to the fact that in some cases the nuclei of the cells of the 

 stomach wall are nearer the inner than the outer euds of the 

 cells, and that in the "basalen Theilen," i.e., the outer portion 

 of the cells, "grijbere und feinere Kornchen ansammeln." 

 Plate XVIII, Fig. 10, shows a section of the stomach wall of 

 an individual with the nuclei at the extreme of migration toward 

 the inner ends of the cells. It will be observed that here they 

 are situated in the inner third or even fourth of the cells. The 

 thicker, clearer state of the outer ends is also seen in the same 

 figure, though the full measure of the structural difference 

 between the two ends is appreciated only by an examination of 

 the stained sections themselves; for the inner, smaller ends take 

 the coloring matter with a good deal more avidity than do the 



