STRUCTURE OP UROCHA3TA AND DICHOGASTER. 249 
increase in size of a few ova. Whatever may be the funda- 
mental explanation of this structural dissimilarity, the fact 
remains that there is a certain difference in the mode of 
development of the ova in the aquatic and in the terrestrial 
Oligochaeta. At the same time it has to be borne in mind that 
in many Earthworms the ova when fully developed leave the 
ovary and make their way to the interior of receptacula ovorum. 
These chambers must at least be analogous to, if not homo- 
logous with, the “ egg-sacs” of Stylaria, &c., in which ova 
also undergo maturation. They differ, however, in being rela- 
tively much smaller and thicker walled, and in having their 
cavity divided up by trabeculae like the vesiculse seminales. 
Yejdovsky does not give a detailed account of the development 
of the egg-sacs (Eiersiicke) in Stylaria, and their homology 
with the receptacula ovorum of Earthworms must be left for 
the present undecided. The question as to homology does not, 
however, affect the functional similarity of the two structures. 
The receptacula ovorum of Earthworms are thicker walled, 
and supplied with abundant blood-capillaries, which give them 
a reddish appearance. The egg-sacs of Stylaria are thin 
walled, and have no capillary network, but are supplied by 
the hypertrophied vascular arch of their segment. This differ- 
ence may perhaps be responsible for the unequal development 
of the contained ova in the two cases. The whole question 
requires further investigation. 
Judging from Bergh’s ( 11 ) figures, the mature ova con- 
tained in the ovary of Lumbricus hardly differ in size from 
those contained in the receptacula ovorum. I have carefully 
compared the relative sizes of the ovarian ova and those from 
the receptaculum ovorum in Alluru s, and find that the latter 
are rather larger; but the difference is not sufficiently striking 
to lead me to the opinion that the ovum undergoes any im- 
portant increase of bulk during its sojourn in the receptaculum. 
Indeed, the observations of Dr. A. Collin ( 14 ) show that in 
Criodrilus the ova contained in the receptaculum are 
smaller than the largest ovarian ova ; but this is probably to 
be explained by supposing that the smaller immature ova ripen 
