6o2 Vernon L. Kellogg. 



multiplying, and on the other hand they modify and even originate 

 those parts of the soma which are lumped together under the name 

 of secondary sexual characters." Again, Wood says, "Much of 

 the variation that we find in male appendages (of Lithocolletids, 

 small moths) is of a neutral character, neither useful nor hurtful 

 to them as clasping organs. All this amazing fertility of shape is 

 dependent in some way upon the presence of the reproductive 

 glands (testes) for it can scarcely be doubted that they could be 

 removed at a sufficiently early date in the life of the larva, the 

 transformation of the last larval segment into the armature of the 

 imago would not occur, much as the emasculation of the deer 

 prevents the development of its horns." 



These positive declarations of Wood are based of course on no 

 evidence except that suggested in the last phrase of the quotation, 

 but are nevertheless of interest as an expression of a problem 

 which has important bearings. To biologists of the epigenetic 

 school holding that the successive phenomena of development 

 find their causative stimuli in the phenomena that have preceded 

 them, the theory that the secondary sexual characters of insects, 

 so numerous and so conspicuous in their divergence in the two 

 sexes, should find a sufficient and immediate stimulus in the 

 pre-development of the primary organs, would be a reasonable 

 one. 



The statement of my experiments with the silkworm moth, 

 Bomhyx mori, to follow, will show that for one species of insect at 

 least this theory can be directly tested. It should be noted that 

 those parts of the adult insect (having a complete metamorphosis, 

 which is the case with the silkworm moth) which show the second- 

 ary sexual characters, as antennae, wings, legs, external genitalia, 

 etc., develop not from corresponding parts in the larva (in most 

 cases no corresponding parts even existing in the larva) but from 

 histoblasts or so-called imaginal discs, which are derived during 

 larval life by the invagination at specific points of the larval skin 

 layer (hypoderm), and which begin to reach in their development 

 the character of the definitive imaginal (adult) parts only in very 

 late larval (pre-pupal) and in pupal life. The primary reproduc- 

 tive organs are on the contrary developed early in larval life (or 



