Umhüllungsmasse." Again, speaking of Daphnia pulex, he describes the youngest eggs as consisting "aus dera Keimblaschen sammt heller Umhüllungsmasse," he uses almost the same words with reference to the young eggs of D. magna; and in speaking of the agamic eggs of D. longispina, he expressly states that he has been unable to ascertain what becomes of the germinal spot after the vitelline mass has quitted the ovary.
These statements seem almost to justify the inference that, after writing the general chapter on the reproduction of Daphnia, he had altered his opinion as to the absence of the Purkinjean vesicle, but had forgotten to modify his statement.
Of course, as we find that some eggs must be impregnated before they can produce an embryo, while others can do so without requiring any external influence, it is clear that these two sorts of eggs cannot be in all respects alike, since identical bodies must have identical properties. Pseudova, however, share all the known essential characters of true eggs; and it would therefore be very interesting to determine, if possible, on what difference of structure this difference of power depends. This, however, is not the only interesting question which we have to solve, since in some cases,—as, for instance, in the hive bee, as just mentioned,—although the egg is capable of development without impregnation, still impregnation exercises an important difference in it (in this case changing the sex of the resulting embryo), and it would be in the highest degree interesting to ascertain how this change is effected.
According to Prof. Leydig, each brood of agamic eggs forms in the ovary one great mass, which is only divided into separate eggs after its entry into the receptacle. It seemed to me, on the contrary, that each egg was produced separately, round a separate Purkinjean vesicle, although no chorion being at first present, and the egg masses being to a certain extent pressed together, the boundary of each egg cannot always be defined.
In Daphnia he describes a vitellarium separate from the germinarium, the former occupying the anterior, the latter the posterior part of the ovary. This is, however (p. 100), not the case in Sida crystallina; and although, as I had already mentioned, the ephippial egg (I believe) always, and the agamic ones very often, arise at the posterior part of the ovary, still I did not observe any separation of the ovary into two parts, so distinct from one another as they are described to be by Prof. Leydig. The various parts constituting the yolk always seemed to me to be developed round the Purkinjean vesicle, as is usual in Crustacea; and we can divide the yolk into "Bildungsdotter" and " Nahrungsdotter" only in the limited sense in which this division holds good for the yolk of all animals.
In Daphnia longispina, Pr. Leydig has made the interesting observation, that from the yoke emerge small vesicles, which he considers to be homologous with the so-called "Richtungsbläschen" so generally present in the development of Mollusca and other animals. This fact,
