576 



Libellulina), is situated a group of large cells. These were first 

 noticed by Heroldt, who described them as rings ; Stein, however, 

 is no doubt correct in asserting that they are the secretors of the 

 yolk, and I therefore agree with Prof. Huxley in calling them vitelli- 

 genous cells. Hermann Meyer, who was followed in this respect by 

 Dr. Allen Thomson, considered them as aborted eggs, an opinion, 

 however, which is quite untenable. A cursory examination of an 

 egg-tube of any Lepidopterous or Hymenopterous insect, will show 

 that although the vitelligenous cells increase individually in size, as 

 does the yolk mass, yet that the latter constantly grows at the expense 

 of the former*, which become gradually fewer in number, and finally 

 disappear altogether. 



Stein has observed, that in Acilius sulcatus, in which the yolk is 

 brightly coloured, the vitelligenous cells are of the same hue. Prof. 

 Huxley has observed in Aphis, and I have found in certain Hemi- 

 ptera, as in Nepa for instance, a canal leading down from the terminal 

 chamber into the egg-tube, and which can be for no other purpose 

 than to convey the yolk matter to the growing eggs. 



Finally, if, as Stein also remarks, we press the vitelligenous cells 

 or nuclei out of one of the egg- chambers, we shall generally find 

 some of them in which the cell-wall is almost entirely absorbed, so 

 that on the application of slight pressure, the contents spread in all 

 directions. 



In its earliest stage, however, the egg-cell cannot be distinguished 

 from the vitelligenous cells, and at the upper part of the egg-tube of 

 any Heteropterous or Dipterous insect, will be found cells, which are 

 neither vitelligenous cells nor egg-cells, but which are apparently 

 capable, under certain circumstances, of becoming either the one or 

 the other. 



Dr. Carpenter has suggested to me that these vitelligenous cells 

 are perhaps analogous to the "yolk-segments" ofPurpura, and this 

 idea throws, I think, some light on the very remarkable phenomena 

 presented by that genus. 



The separation of the egg-germs from the vitelligenous organs, is 

 a condition found in many worms, in some Crustacea, as for instance 

 in Cyclops, and probably in the Cirrhipedes. 



Our knowledge of the modes of egg-formation in the Insecta, is 

 * See Lacordaire, Introd. a 1'Entomologie, v. ii. p. 386. 



