636 DAVID HILT TENNENT. 
From this rather limited field my research gradually 
broadened until it included not only the study of the germ- 
cells, but also that of the older cercaria stages, as well as 
investigations along various lines for the purpose of deter- 
mining the complete life history of the form under considera- 
tion. 
This work, of which the present paper is an account, has 
been carried on in the Zoological Laboratory of the Johns 
Hopkins University, and in the Fisheries Laboratory at 
Beaufort, North Carolina. 
It is with pleasure that I acknowledge my indebtedness to 
the Honourable George M. Bowers, United States Commis- 
sioner of Fish and Fisheries, for the opportunity of enjoying 
the advantages of the Beaufort Laboratory, and to Dr. 
Caswell Grave, Director of the Laboratory, by whom many 
courtesies and privileges have been extended. To Mr. R. H. 
Coker I am indebted for material from Pamlico Sound, and 
to Mr. O. C. Glaser for infected oysters from Cameron, 
Louisiana. 
To Professor Brooks I express my most hearty appreciation 
of the unfailing interest, and of the many kindnesses which 
have characterised his direction of my work. 
MATERIAL AND MeEtTHODs. 
The abundance of infected oysters at Beaufort provided 
the opportunity of making extended observations on living 
material. ‘That this has been done will be apparent from the 
account of my work. 
For the continuation of my work in Baltimore, recourse 
was had to preserved material. In my work on ‘ Bucephalus’ 
I made use of many fixing agents. Corrosive sublimate 
solution, either with or without the addition of acetic acid, 
picric acid liquids, and Perenyi’s fluid gave extremely poor 
results. ‘Tissues placed in Gilson’s mixture for two hours 
were excellently fixed and gave very useful preparations. 
By far the best fixation, however, was obtained by the use of 
