226 MONTGOMERY A STUDY OF THE CHROMOSOMES 



But from this it would follow that the heterotypic mitosis of the first spertnatocytes 

 of the Vertebrates is really a reduction division, and results in the separation of whole 

 univalent chromosomes. Then the longitudinal split of such bivalent chromosomes would 

 be really the space between two univalent chromosomes. Thus, though these chromosomes 

 may appear in the prophases longitudinally split, yet a separation of the daughter chromo- 

 somes along the line of this split would not be an equational division. The workers on 

 vertebrate spermatogenesis have indeed shown that the bivalent chromosomes are split 

 longitudinally, but since none of them have succeeded in demonstrating h<>\v the bivalent 

 chromosomes are formed in the synapsis stage they could not show the ni^iiil'icance of 

 this longitudinal split. For Peripatus and the Hemiptera I have shown that a bivalent 

 chromosome is produced by one end of one univalent chromosome uniting with one end 

 of another ; while in the Vertebrata, if my interpretation is correct, a bivalent chromosome 

 would be produced by the union of both ends of one univalent chromosome with both 

 ends of another the spermatogonic chromosome is U-shaped, the spermatocytic chromo- 

 some is ring-shaped since it represents two such U-shaped elements with their ends coii- 

 nected. Also in Peripatus and the Hemiptera there are occasionally ring-shaped chro- 

 mosomes similar to the heterotypic chromosomes of Vertebrates, and they are formed by 

 the two ends of one univalent chromosome being joined with the two ends of another, 

 instead of one end being joined simply with one end. 



This interpretation explains, and the process has never been satisfactorily explained 

 before, why one of the maturation mitoses in Vertebrates is heterotypic : it is a reduction 

 division separating entire univalent chromosomes, and it differs from all other mitoses 

 of the germinal cycle because it is the only one of them which does separate entire chro- 

 mosomes. If this view is true, then probably all Metazoa would have in common the 

 occurrence of one reduction division, and we should no longer be confronted by the dis- 

 crepancy between Metazoa with and those without a reduction division. The occurrence 

 of a reduction division is actually proved for the Copepoda, Insecta, Oniseus and Peri- 

 patus (I shall not mention other objects where it has been rendered probable but not 

 thoroughly proven) ; a priori we should expect that one would occur in the Vertebrates 

 also, and in the Vertebrates there does occur a peculiar heterotypic division which, as 

 we have seen, can be satisfactorily explained as a reduction division. Accordingly, the 

 term " heterotypic mitosis" might be applied to any mitosis which results in the separation 

 of whole univalent chromosomes, irrespective whether it divides the bivalent chromosome 

 transversely or longitudinally ; the term " heterotypic " is indeed most excellent in that 

 it expresses a mitosis " of a type of a different kind," one differing from all other mitoses 

 of the germinal cycle. Of very secondary importance, then, would be the form of the 

 chromosomes not the form but the way in which the chromosomes divide should be 



