Miscellaneous. 497 



larva), these cells, on the contrary, separate speedily to a certain ex- 

 tent from the adipose tissue in OHgarces (the larva of Pagenstecher 

 and Leuckart), although they do not, asLeuckart maintains, constitute 

 a true ovary either in Miastor or in OHgarces. In fact, all the cells 

 become developed into ova and larvae, and none of them serves for 

 the formation of the stroma, for the formation of the envelopes of 

 the ova, or for any other analogous purpose. 



In order to explain the peculiarities of these animals, I have en- 

 deavoured to establish a theory of the formation and development of 

 the ova in the whole animal kingdom, of which the following is an 

 abridgemnt. 



The ovum is composed either of a single cell, " the germinal cell," 

 or of the germinal cell accompanied by several other " vitelline cells," 

 or by the secretion of the latter, " the vitelline mass." The ovum 

 of the Mammalia, and that of most of the inferior animals, belongs to 

 the first category ; that of other animals, and especially that of 

 Birds, belongs to the second ; and that of most Insects to the third 

 kind. The "germinal cell" alone, the nucleus of which is the 

 " germinal vesicle," is subject to the vitelline segmentation which is 

 so often discussed. The "vitelline cells" and the "viteUine mass" 

 are not segmented, but pass, without any other form of development, 

 into the nutritive vitellus. The germinal cell divides by segmentation 

 into minute cells (embryonal cells). A portion of these, not absorbed 

 by the formation of the embryo, furnish material for the new ovaries 

 and testes, inasmuch as in general some of the cells form a stroma 

 which separates and encloses a greater or smaller quantity of the 

 other cells. The remaining non-separated cells form, in Insects, 

 what is called the adipose tissue. 



A second element, the semen, is necessary in most animals, to en- 

 able the ovum, or rather the germinal cell of the ovum, to develope 

 itself; but this stimulus is not always necessary in a great number 

 of the inferior animals. The development of the ovum without 

 stimulus or fecundation is by no means dependent upon a certain 

 more or less advanced point of development of the maternal animal, 

 or of its ovary ; for sometimes the maternal animal attains a com- 

 plete development even with external and internal genital characters 

 (parthenogenesis, as in the bee), sometimes it propagates only in 

 the state of a larva without genital characters, and this may be re- 

 peated through several generations, either under the same larval 

 form (as in our Cecidomyides) or under a different exterior form 

 (alternate generations or metagenesis — Trematoda). I by no means 

 assume that there is any well-marked limit between parthenogenesis 

 and metagenesis ; for example, the mode of reproduction in the 

 Aphides might be explained in both ways. 



As compared with other insects, I also regard it as characteristic 

 that, whilst in general we must make a distinction between the epi- 

 thelial and vitelline cells, and the latter serve only for the nourish- 

 ment of the embryo, in the present case the epithelial cells serve at 

 once as epithelium and as vitelline cell" *" ♦v^-^ lorvAp. 



