Page:Natural History Review (1861).djvu/37

Rh if correctly interpreted, is very interesting, since the occurrence of the directive vesicle in groups so essentially distinct would tend to prove that it is a structure of importance, and not a mere particle of the yolk excluded to give more space to the rest. It is also important, because it forms another bond of union between two groups so widely separated as are the Mollusca and the Articulata.

Another point of great importance is the determination of the presence of yolk-division in the egg. Up to the present time, this process had been satisfactorily determined, so far as the Crustacea were concerned, only in certain Cyclopidæ and Ergasilidæ, to which, apparently, we may now add Daphnia brachiata and Polyphemus oculus, both of which species seem, from the transparency of their yolk to be specially well adapted for embryological examination.

From finding many empty eggshells in the receptacles of specimens, which also contain healthy embryos, Prof. Leydig infers that, as a rule, many of the agamic eggs perish during their development; but the observation need not lead to any such conclusion, since the eggshells are always cast off at a very early stage; and the embryos, when first hatched, have, from their oval form, been already, by many observers, confounded with the true agamic eggs.

Prof. Leydig brings no farther evidence to bear on the curious fact, confirmed by all my experience, that all the young of each brood are of the same sex. On this point I have accumulated a good deal of information. Not only did I carefully watch the origin and development of many broods in the summer of 1856, but between the 1st June and the 11th December, 1858, I obtained nine generations of Daphnia, all of which, except perhaps the first, were indubitably produced from agamic eggs. As this experiment is of much interest, I may perhaps be per- mitted to give the details.

On the 22nd of June I took two young Daphnias, the mother of which had been born by a specimen isolated in a tumbler. Neither these, nor their mother, nor grandmother, had ever been in a glass with a male; but I have not kept the date of their birth,

On the 4th of July these had young ones, some of which I put in another glass. These, therefore, formed the fourth generation without impregnation.

On the 21st of July these again bore young ones; and again on the 29th, some of which latter I isolated.

On the 19th of August these again produced young, which I isolated.

On the 3rd of September,

On the 1st of October,

On the 1 st of December these again had young ones, which died without breeding.

During this experiment I carefully examined all the young ones produced; and as the males are, even directly after birth, easily distinguishable from the females, I think I can undertake to say that not one of the latter was produced during the whole time. This series of