212 
G. AUNOLD. 
are yellow. Apart from this, fat-globules can be seen in the 
cytoplasm of the endotlerm cells, far too large to be mistaken 
for any corpuscle lying over or under one of those cells. It 
must be remembered that no ingestion of the corpuscles takes 
place until some considerable time after the one hour stage, 
at which these Planaria were killed. 
This fact indicates, at least, that the fat which appears in 
the endoderm cells is a neutral fat, but whether the secretion 
of the gland cells breaks down the fat of the blood into fatty 
acids could not be ascertained, for the colour of the blood- 
corpuscles completely masks any blueness which might be 
present in the food magma. 
However, sections of the one and a half hour stage, cut 
in paraffin and stained with Nile-blue sulphate, showed a 
definite bluish tinge in the magma, but not a trace of red. 
The significance of this fact is important, for it shows that 
the digestive process in Planaria is not, as has been stated by 
Metschnikoff (‘ L’lmmunite,’ 1902), entirely intra-cellular, and 
at the same time indicates the first step in the formation of 
the highly complex digestive apparatus found in the higher 
animals. 
This first step is, we have seen, the production of a secre- 
tion by certain cells which enables fat to be absorbed. Such 
cells are unicellular glands. If during the course of evolu- 
tion these unicellular glands, instead of being difl^used 
throughout the intestine, become aggregated in certain areas, 
we are enabled to picture the formation of any of the multi- 
cellular glands which line the intestinal tract by the 
subsequent invagination and enlargement. 
Metschnikoff (’02) and in his work ‘ L’lmmunite dans les 
(Maladies Infecteuses,’ says that in Planaria digestion is 
entirely intra-cellular, and this seems hitherto to have been 
widely accepted. 
Mesnil (’01) comes to the same conclusion in regard to the 
Actinia, but this disagi-ees with the results of several other 
workers. 
Pratt’s (’05) observations on the digestive organs of the 
