THE SPERMATOGENESIS OF UMBRA LIMI. 135 



in the shape of L's and Vs. Many of these chromosomes go 

 through a contraction at the apices of the V's until small trans- 

 versely constricted chromosomes are formed. Discussing the 

 contraction phenomenon, Agar ('12), says: " It becomes operative 

 especially, but not solely, whenever the chromosomes are short 

 in comparison with their length, as happens normally in meiosis 

 and exceptionally in somatic tissues." I find in Umbra, as 

 has already been stated, numbers of spermatogonial cells showing 

 varying degrees of contraction; this contraction is apparently 

 toward the apices of the V's, as Agar has already pointed out. 

 Early metaphase or late prophase (Fig. 5) in Umbra may occa- 

 sionally show very Jong chromosomes with little or no evidences 

 of contraction. Different cells show late metaphases and early 

 anaphases in varying degrees of contraction. Anaphases may 

 be found, although quite rarely, with chromosomes similar to 

 those represented in Fig. 14. A more contracted state than that 

 shown in Fig. 5, is probably the normal condition arrived at in 

 metaphase. Extreme contraction, as shown in Figs. 12 to 14, 

 are found in medium sized cells embedded between larger cells, 

 in the regions of the testes where most connective tissue abounds, 

 and at the periphery of the lobules. This extreme contraction is, 

 I believe, the unusual rather than the usual occurrence. This 

 condition as well as those that have been previously described, 

 I interpret as being due to relative growth rates of the cells. 

 Cells dividing rapidly do not show contraction to any appreciable 

 extent while those dividing more slowly and remaining in each 

 phase of mitosis longer, exhibit it more markedly. The slow 

 growth in the extremely contracted phase may be due to tension 

 exerted upon such cells by the connective tissue surrounding 

 them, or by the larger and more actively growing neighbor cells. 

 Contraction, however, is to a certain degree a regular procedure, 

 and the shorter chromosomes, as w r as found by Agar in Lepi- 

 dosiren, contract first and most. In Umbra, however, contrary 

 to the view of Agar, I am inclined to believe that these trans- 

 versely constricted chromosomes play no considerable part in 

 meiosis. 



Some investigators have reported a definite growth period, 

 others no such period, in the primary spermatocyte divisions of 



