196 PROCEEDINGS OF THE ACADEMY OF [Feb., 



two in close union, but not so close as to prevent their separation in the 

 reduction mitosis. There is some observational proof for this, in that 

 the odd chromosome or unpaired heterochromosome in the spermato- 

 gonium sometimes exhibits a transverse constriction, as if marking the 

 point where two had joined, in Harmostes and Protenor; and in Pro- 

 tenor the division of the heterochromosome in the reduction mitosis is 

 at right angles to its long axis. The failure to divide in the second mi- 

 tosis can only be ascribed to an incomplete process of longitudinal 

 splitting during the growth period. We can thus express the likeness 

 and difference between the single heterochromosomcs and odd ordinary- 

 chromosomes of the Hemiptera and the single heterochromosomes 

 of the Orthoptera; they all agree in dividing reductionally in 

 the fu-st maturation mitosis, whether by a separation of two uni- 

 valent components or by a transport of the whole chromosome into 

 one of the daughter cells; they differ merely in not undergoing or in 

 undergoing an equational splitting in the second mitosis. We can 

 sum this up in the statement: all chromosomes and heterochromo- 

 somes, be they paired or single in the spermatogonJa, divide reduction- 

 ally in the first maturation mitosis, whether this division consist in two 

 univalent components separating from each other or a single compo- 

 nent passing undivided into one of the second spermatocytes. 



And now we come to another point with regard to a general uniform- 

 ity of heterochromosomes. I first showed (1901a, h) that the ordinary 

 chromosomes in the spermatogonia are arranged in pairs, so that, e.g., 

 fom-teen chromosomes form seven pairs, the two of a pair being alike in 

 size ; and I showed for several species that w^henever spermatogonial 

 chromosomes show marked differences in size they can be recognized 

 again in the bivalent chromosomes of the spermatocytes. Sutton (1902) 

 corroborated this for Brachystola.* And later I showed (1904a) cor- 

 responding chromosomes in the spermatogonia are alike not only in 

 size but also in form. We have just seen, also, that one kind of hetero- 

 chromosomes, the chromatin nucleoh, occur in pairs in the spermato- 

 gonia — where there Is one bivalent one of these in the spermatoc}i:es it 

 corresponds to two in the spermatogonia. Further than this, we have 

 shown that the odd ordinary chromosomes of Hemiptera and the 

 unpaired heterochromosomes of Protenor must be regarded as already 



* Boveri (1904), in his recent re\aew, ascribes the main credit of this discovery 

 to Sutton (1902),' as others have done; but the point was very clearly stated in 

 my papers, illustrated on a number of species, and, furthermore, I demonstrated 

 that chromosomes of corresponding size conjugate to form the bivalent ones. 

 Quite a number of papers have come out recently with " new discoveries " 

 which had already been made in my papers on Peripatus and Hemiptera. 



