24 REVIEWS. 



TJmhiillungsmasse.'' Again, speaking of Daphnia pulex, he describes 

 the youngest eggs as consisting "aus dera Keimblaschen sammt heller 

 Umhiillungsmasse," 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 

 o^ary. 



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 proper- 

 ties. 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 de- 

 pends. 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 " Bildungsd otter" 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 obser- 

 vation, that from the yoke emerge small vesicles, which he considers to 

 be homologous with the so-called " Richtungsblaschen" so generally 

 present in the development of Mollusca and other animals. This fact, 



