194 W. E. AGAR 



conspicuous as in M a c r o p u s, probably because the X and Y 

 chromosomes are more nearly of the same size. Indeed, for 

 a long time I thought there was no visible difference between 

 the two types, ])ut closer examination has shown that such 

 a distinction exists. All secondary spermatocytes have a com- 

 pact chromatic nucleolus, but in the case of young sister nuclei, 

 whose relation to each other can still be seen by the persistent 

 spindle remains, one of them constantly has a distinctly larger 

 nucleolus than the other. Sometimes, as in the pair figured 

 (PI. 14, fig. 37), this is expressed by one of them being bilobed 

 and the other single. In other cases it is merely a difference in 

 size. The difference between the two classes of secondary 

 spermatocytes is thus very small, but once recognized it is seen 

 to be constant. 



Discussion of some Special Problems raised by the 

 foregoing descriptions. 



(1) Chro mo meres. — Many cytologists maintain that 

 chromomeres are purely artefacts, due to unequal contraction 

 of the chromatic thread under the influence of the fixative, 

 or else, in the case of smaller chromomeres, are mere optical 

 effects of angles, &c., in the thread. At any rate, according to 

 this view, they do not represent any real local differentiations 

 of the substance of the chromosome. 



The strength of the criticism that the chromomeres are 

 artefacts depends much upon the exact meaning attached to 

 that word. It is of little importance whether or not chromo- 

 somes which are beaded when fixed appear smooth in life (a very 

 difficult observation in any case !) For the sake of argument 

 it may be granted that the beading is produced by the action 

 of the fixative. The important question is : Is the beading of 

 such a nature that it could be produced mechanically by 

 precipitation in and contraction of a homogeneous thread, or 

 is it the expression of a pre-existing though perhaps invisible 

 differentiation in the living chromosome ? Doubtless so-called 

 chromomeres have been described which might have been 



