No. .-342] 



SOME ASPECTS OF CYTOLOGY 



63 



that the spireme-threads often consist of linear series of 

 grannies. I have in mind the fact that the number and 

 size-relations of the chromosomes often differ materially 

 as between different species, even nearly related ones, and 

 that in at least one case (that of the X-chromosome) it is 

 an established fact that a particular chromosome which 

 in some species is a single body may be represented in 

 other species by two or more components that sometimes 

 show constant and characteristic di (Terences of size. The 

 natural interpretation of this fact is that the chromo- 

 somes are compound bodies, consisting of different con- 

 stituents which undergo different modes of segregation 

 in different species. We may here find a rational expla- 

 nation, both of sex-limited heredity, as I have elsewhere 

 indicated, and of other kinds of coupling. 



2. Recent studies on karyokinesis and maturation em- 

 phasize anew the importance of the mitotic transforma- 

 tion of the chromatin-snhstance, and add weight to 

 Roux's original interpretation of this phenomenon. 

 Nothing in recent cytological research is more interest- 

 ing than the discovery by Bonnevie, Pinney, Davis, and 

 others that new chromosomes may arise within the old 

 ones in the form of tightly coiled or convoluted threads, 

 which uncoil or unravel to form separate spireme-threads. 

 In karyokinetic division these threads may be formed 

 already inside the telophase-chromosomes of the preced- 

 ing division, as was discovered by Bonnevie. In other 

 cases, an example of which is given by certain Orthoptera 

 first reported upon by Miss Pinney, they are first visible 

 in the early prophases, when they are seen uncoiling 

 from massive bodies formed from the old chromosomes 

 and equal in number to them. The same remarkable 

 process occurs in the early auxocytes, as the chromosomes 

 are preparing for conjugation in synapsis, as has been 

 shown particularly by Davis, whose observations, like 

 those of Pinney, I have recently been able to confirm 

 and extend. In many insects the presynaptic spireme- 

 threads do not arise, as has often been described in other 



