NOTES AND MEMOKANDA. 413 



leaves of Bromelias, the pupae have any necessity, nor would even he 

 able, to swim, and in the four species living in such localities which 

 he examined, and which belong to as many different families, the feet 

 of the pupfE are quite hairless, or nearly so, while in allied species 

 of the same families or even genera (Helicopsyche) the fringes of the 

 legs, used for swimming, are well developed. 



This abortion of the useless fringes is of considerable interest, 

 because it cannot be considered, as in many other cases, as a direct 

 consequence of disuse ; for at the time when the pupfe leave their 

 cases and when the fringes of their feet are proving either useful or 

 useless, these fringes, as well as the whole skin of the pupa, ready to 

 be shed, have no connection whatever with the body of the insect ; it 

 is therefore imjiossible that the circumstance of the fringes being 

 used or not for swimming, should have any influence on their being 

 developed or not developed in the descendants of these insects. As 

 far as he can see, the fringes, though useless, would do no harm to the 

 species, in which they have disappeared, and the material saved by 

 their not being developed appears to be quite insignificant, so that 

 natural selection can hardly have come into play in this case. The 

 fringes might disappear casually in some individuals ; but, without 

 selection, this casual variation would have no chance to prevail. 

 There must be some constant cause leading to this rapid abortion of 

 the fringes on the feet of the pupte in all those species in which they 

 have become useless, and he thinks this may be atavism. For caddis- 

 flies, no doubt^ are descended from ancestors which did not live in the 

 water, and the pupte of which had no fringes on their feet. Thus 

 there may even now exist in all caddis-flies an ancestral tendency to 

 the production of hairless feet in the pupae, which tendency in the 

 common species is victoriously counteracted by natural selection, for 

 any pupa, unable to swim, would be mercilessly drowned. But as 

 soon as swimming is not required and the fringes consequently become 

 useless, this ancestral tendency, not counterbalanced by natural 

 selection, will prevail, and lead to the abortion of the fringes. 



Comparative Embryology of the Insecta.— Professor Graber in a 

 preliminary article * gives a brief history of the results of his 

 observations, which appear to be of considerable importance. An 

 examination of the ovarian cell at an early period has revealed the 

 presence, in the centre of the yolk, of a number of amoeboid cells, 

 which appear to have been formed by the division of the germinal 

 vesicle ; these " primary embryonic cells " have a relatively large 

 nucleus and a number of nucleoli ; several may be seen to unite with 

 one another by means of their pseudopodia, and they may also be 

 observed to undergo division. The blastosi^here always consists of a 

 single cell - layer, and always undergoes " emboly " ; its first dif- 

 ferentiation is into two layers only, or, in other words, there is no 

 independent appearance of the mesoderm, which in these forms at any 

 rate always owes its origin to the endoderm (endoblast). The 

 internal germinal cells arise in two ways, some independently of the 



* 'Arch. f. Mikr. Anat.,' xv. (1878) 630. 



