144 GEORGE W. TANNREUTHER 
INTRODUCTION 
The Discodrilidae are, in many ways, extremely favorable 
for the study of annelid development. The material can be 
readily obtained at almost any season of the year. The develop- 
ment of any one egg can be followed from the time of fertiliza- 
tion to its complete development. The smallness of the eggs 
and the chitinous-like cocoon are the most objectionable features 
to contend with. Notwithstanding these facts, this group of 
annelids has been almost completely neglected by investigators 
in the study of cell lineage. 
Moore, in his paper on “The anatomy of an American Dis- 
eodrilid (Bdellodrilus illuminatus),” refers incidently to his 
‘embryological studies’ in the course of his investigations, but 
has published no account of the Discodrilid development. 
Salensky, in his paper on “The development of Branchio- 
bdella an European Discodrilid, parasitic on Astacus fluvitalis,”’ 
gives an account of the cleavage, axial relation, origin of the 
germ layers and the formation of the adult structures. But the 
points of chief importance are so inadequately described and 
imperfectly worked out, that his results have no special signifi- 
cance in the study of Discodrilid development. 
The development of Bdellodrilus philadelphicus, one of the 
Discodrilidae, has, so far as I can learn, never been worked out. 
It is in many respects a very important form, not only from the 
standpoint of development, but from its adult anatomical strue- 
ture as well. 
The more important points in the following paper may be 
briefly summarized as follows: 
1. The cleavage is unequal and regular, but may be variable 
in some eggs. A very small cleavage cavity is present. The 
gastrula is formed by the epibolic process. The blastopore 
occupies a very small area on the ventral surface. Its point 
of closure corresponds to the median ventral side of the adult 
worm. 
2. The early cleavage planes are definitely related to the 
future organs of the adult worm; i.e., every cleavage foreshadows 
