THE GERM-PLASM 87 



nucleus in this animal contains two or four rod-like chromosomes 

 (see fig. 2), each of which is composed of '' six thickened granu- 

 lar or disc-shaped portions, which become deeply stained with 

 colouring matter, and which are separated by portions staining 

 less deeply ' (Boveri). If we connect this fact with the hypo- 

 thetical composition of the germ-plasm out of ids, it follows 

 that an id cannot in any case be larger, and is probably smaller, 

 than one of these granules or microsomata. It cannot be 

 larger, because the id is a unit which is capable of division into 

 two daughter-ids, but which cannot remain permanently sepa- 

 rated into several parts by a different kind of intermediate sub- 

 stance. If we suppose the id to be as large as it can possiblv 

 be, — that is to say, to correspond in size to a microsome, — it 

 will measure, according to Boveri's drawing and scale of enlarge- 

 ment, .0,008 mm., or not quite i m in diameter. Only the ter- 

 minal granules of the rods, however, are as large as this ; the 

 greatest diameter of those in the middle measures .0.006 mm., 

 while their shorter diameter is about .0.003 — -0.004 m.n^- The 

 terminal granules, looked upon as spherical bodies, would be 

 capable of containing about two million biophors of the size 

 given above. 



This number is certainly a very considerable one, and it would 

 apparently be sufficient to make up the number of determinants 

 in such a lowlv organised animal as Ascaris. But even 



& 



m 



Arthropods the number of determinates, and therefore that of 

 the determinants also, is considerably greater. Each of the 

 olfactory sets on the feelers of Crustaceans, which were 

 mentioned above, must be capable of being determined from the 

 germ onwards ; and this is also true of the spots and lines on 

 a butterfly's wing, each of which represents at least one deter- 

 minant, and in case of all the large markings several, or even 

 many, of these units. If we consider that the pattern on the 

 wing is often very complicated, and frequently differs on the 

 under and upper surfaces, it is evident that hundreds of deter- 

 minants must exist for this pattern alone. But there are, again, 

 several peculiarities in the structure of the wing-scales, and thus 

 it is probable that almost every scale can vary independently 

 from the germ onwards. In some males of the family Lycaentdce, 

 e.g., Lycaena adonis, small guitar-shaped odoriferous scales (the 

 'androconia' of Scudder) are distributed regularly amongst 

 the colour-scales, while these are entirely absent in other nearly 



