v] SEGREGATION OF CHROMOSOMES 73 



Lepidosiren have a four-fold appearance, closely resembling 

 typical tetrads, due to the constriction of each single 

 chromosome in the middle, but this constriction in no way 

 corresponds with a subsequent plane of division. Similarly 

 the transverse break which has been assumed to be the point 

 of attachment of the single ("univalent") chromosomes 

 to each other at the first appearance of the gemini in Cope- 

 pods is quite possibly a constriction of the same kind, and 

 these bivalents may thus with great probability be regarded 

 as consisting of pairs united side-to-side, rather than end- 

 to-end. 



Whatever view is taken of the origin of the gemini or 

 bivalents, it is clear that the final result consists in the 

 separation of single chromosomes (univalents) from one 

 another in one division, followed (or possibly in rare cases 

 preceded) by a longitudinal splitting of these single chromo- 

 somes in the other. Since this result is the same whether 

 the bivalents are formed by side-to-side or end-to-end 

 pairing, it may seem unnecessary to devote so much space 

 to the discussion of that part of the process, but, as will be 

 seen in a later chapter, very important theoretical specula- 

 tions have been based on the assumption that side-to-side 

 pairing normally takes place, and these speculations would 

 be meaningless if end-to-end pairing were in fact the rule. 



Some objections that have been brought against the 

 generally accepted hypothesis that the "reduction division" 

 separates complete chromosomes from each other will be 

 considered in a subsequent chapter. 



