Development of Dreissena polymorpha, Pallas. 161 
Lake Tegel as also in that of the mussels which are pre- 
served under the most favourable possible conditions of exist- 
ence in the reservoir of the garden of the Zoological Institute 
here. In the case of two of the latter, which had been 
obtained from 'Tegel only a short time before and placed in 
the reservoir, I observed as early as the middle of May the 
deposition of a large number of ova, which were found in 
each instance in a cohering mass near the mussel. These 
eges did not develop, but soon perished. It was not until 
the middle of June that the deposition of ova was observed 
once more, and shortly after this the normal reproduction 
and development commenced. Whether it was merely retarded 
this year in consequence of the peculiarly low temperature of 
the spring, or whether it always takes place at this relatively 
late season, can only be determined by the observations of 
future years. 
The very small ova of Dretssena polymorpha are deposited 
freely in the water; they are only surrounded by an extremely 
delicate envelope and are very scantily supplied with yolk. 
This very condition of the ova was evidence of the occur- 
rence of free-roving larve. ‘he method of oviposition is as 
follows. The mussel slightly opens the valves of its shell, 
and, quickly closing them again, a little ball of ova is 
extruded. ‘This process is repeated several times and is 
readily observable in the aquarium by arranging the proper 
time. The balls of ova, which do not at once lose their 
cohesion, appear like little whitish lumps of mucus. 
The unequal segmentation of the ova exhibits a great 
agreement with that of other mussels, as described by Lovén, 
Flemming, Rabl, Hatschek, Ziegler, Horst *, and others. 
The same is also true on the whole for the subsequent develop- 
mental stages, which will receive a detailed description at a 
later date, and will here be considered only in so far as they 
are necessary for the comprehension of the development of 
the larval form. ‘These processes present a quite peculiarly 
close approximation to the conditions which obtain among 
the marine mussels, which is not to be wondered at for the 
very reason that they soon lead to the same result. 
An expansion of the primitively narrow segmentation- 
* W. Flemming, “Studien in der Entwicklunesgeschichte der Na-~ 
jaden,” Sitzungs-Ber. Akad. Wiss. Wien, Math.-nat. Cl. 51 Bd., 1875, 
C. Rabl, “Ueber die Entwicklungsgeschichte der Malermuschel,” 
Jenaische Zeitschr. Naturw. 10 Bd., 1876. 
R. Horst, “ Embryogénie de Vhuitre (Ostrea edulis) ,” Tijdschrift Neders 
landsche Dierkundige Vereenigung, Supplement, Deel i., 1883-84. 
“On the Development of the Kuropean Oyster,” Quart. Journ. Mier. 
Sc. vol. xxii., 1882. 
Ann. & Mag. 'N. Hist. Ser. 6. Vol. ix. 11 
