48 GEGENBAUR, ON SAGITTA. 
from the oral hooklets. From two of these three distinct 
species Gegenbaur obtaied mature ova, derived from preg- 
nant individuals kept for the purpose in glass vessels, 
The spawn was deposited in good-sized masses of a sub- 
stance not unlike swollen sago-grains. The period at which 
they were most abundantly met with extended from the end 
of January to the beginning of March. 
The deposited spawn always lay unattached on the bottom 
of the glass vessel, and consequently when in the sea is pro- 
bably pelagic, that is to say, the sport of the waves. In 
confirmation of which similar masses of ova were occasionally 
taken with a fine towing-net. They bere no resemblance to 
the ova described by Darwin as belonging to Sagitta. 
The ova were enveloped in a gelatinous substance, which, 
however, did not appear to surround each ovum separately, 
but to appertain to the whole mass of eggs in common. In 
this respect some resemblance may be observed with the 
condition presented in Terebella, Protula, and <Arenicola, 
as well also as in the Hirudinee and those Lumbricina in 
which several ova are associated in one capsule (e. g., 
Senuris). 
The spawn of Sagitta, therefore, is closely allied to that of 
Annelids, and differs essentially from that of the Mollusca, 
and especially of the Gasteropoda, whose ova, besides the 
general gelatinous envelope, present also an albuminous 
covering surrounding each separate vitellus or several toge- 
ther, when the outer layer is hardened into a membranous 
case. 
The size of the ova varies according to the species. Those 
of the small species measure /,", and of the larger 1”. In 
other respects they are alike, perfectly spherical, and almost 
perfectly pellucid, with a shghtly yellowish tinge, and fur- 
nished with an extremely delicate vitellme membrane. 
In the centre of the yelk lies the resistant, yellowish ger- 
minal vesicle (nucleus), having a diameter about th of that 
of the vitellus. No germinal spots (nucleoli) were noticed. 
Before segmentation has taken place, the vitellus appears to 
be composed of a perfectly homogeneous substance contain- 
ing minute molecules, which are more closely crowded 
towards the centre than in the peripheral portion. The 
segmentation of the yelk, as well as the entire process of 
development, is quickly ended, occupying the space of seven 
to nine days. ‘The process of development was the same in 
the ova of both the species observed. 
Segmentation commences with the appearance of a groove 
following the equatorial line of thevitellus, but beneath the vitel- 
