GERM CELLS OF COELENTERATES 385 
Material killed in the mixture of Bouin seems to be especially 
favorable for study, showing in fine detail even very delicate 
structures; on the whole the fixation from this mixture seems to 
behave better for coelenterates than any other I have used. One 
can depend on the results given by it, since a close comparison 
of results obtained from this and from other fluids, as mercuric 
chloride, osmic acid, picric acid, potassium bichromate, platinic 
chloride, and so forth, show no essential differences, it being 
mainly a difference as regards staining. The very granular 
appearance found after the use of mercuric chloride mixtures, 
however, is in large part artificial, due to the vigorous precipita- 
* tion of the colloid substances by the mercury. The material 
killed in Bouin’s fluid has been used more than any other, though 
the results have always:been checked by reference to other methods 
of fixation. 
I should like to call especial attention to the fact that during 
the study of this form there has been a study of living material 
as well. Of course there are many features that could not be 
seen in the living eggs, but a great many structures of the nucleus 
and of the cytoplasm are almost as clear in the living material as 
in the sectioned and stained preparations. To check the work on 
Campanularia, living eggs of Obelia were also observed and the 
conditions found are the same as the ones figured for Campanu- 
laria. The sections show that all these features observed in life 
were faithfully preserved and clearly shown, and one may con- 
clude, therefore, that the things not seen in life have probably 
been well preserved also. 
In the paper on the tubularian hydroids I discussed the matter 
of stains; some or all of these have been used here. Heidenhain’s 
iron-hematoxylin, as might be expected, has given the best results 
in the delineation of the form of the nuclear and cellular constit- 
uents. Combinations of various sorts have been used to deter- 
mine; as far as possible, the similarity or dissimilarity of various 
constituents and their possible significance, origin and fate. The 
sections were usually cut 5 to 7 uw thick in order to have the 
nucleus in few sections, with the result that where chromosomes 
were present they usually appeared in a single section. 
