Z>eye/o^:)men^ o/'Dreissena poljmoi-pha, 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 

 liere. In the case of two of the latter, which had been 

 obtained from Tegel only a siiort 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 

 eggs 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 Dreissena 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 larvaj. The method of oviposition is as 

 follows. The mussel slightly opens the valves of its shell, 

 ami, quickly closing tliem 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 Loven, 

 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. Flemmmg, " Studieu iu der EtitwickluDgsgescliichte der Na- 

 jadeu,'' Sitziings-Ber. Akad, Wiss. Wien, Math.-uat. 01. 51 Bd., 1875. 



C. Ra'ol, " Ueber die Eutwickluiigsgescliichte der Malerinuschel," 

 Jenaisclie Zeitschr. Naturw. 10 Bd., 1876. 



K. Horst, " Embrjogeuie de I'huitre [Ostrea edidis)," Tijdsclirift Neder- 

 landscbe Bierkundige Vereeuiguug, tSupplemeut, J)eel i., 188-3-84:. 



" On tbe Bevelopiiieut of tbe European Oyster," Quart. Jouru, Micr. 

 Sc. vol. xxii., 1882. 



Ann. tf' Mag. X. Hist. Ser. 6. Vol. ix. 11 



