CHANGES IN REPRODUCTIVE CELLS OF ELASMOBRANCHS. 293 
the rudimentary flagellum is withdrawn, but the centrosomes 
remain immediately beneath the membrane of the cell. 
VI. The Last or Second Heterotype Phase of the 
Second Spermatogenetic Period. 
38. The last division in the second spermatogenetic period 
is a heterotype, like the first, but the elements are scarcely 
more than half the size. The number of the chromosomes is 
again twelve, and, like those of the earlier divisions, they 
become eventually grouped together as a globular mass in the 
centre of the nuclear sap (figs. 66, 67, and 68). 
When the spindle has been formed, the ring chromosomes are 
not altogether on its surface, and, owing to their small size, a 
polar view of the monastral figure often presents the curious 
appearance represented in fig. 70, on account of the upper 
and lower edges being in focus at the same time. The chro- 
matic loops eventually split transversely, like those of the 
previous division, the daughter V’s travelling to the pole in the 
manner represented in figs. 69, 71, and 72. 
Thus the last division in the spermatogenesis of these fishes 
is, as I pointed out so early as in 1893,! a perfectly normal 
affair, each mother-chromosome splitting into two daughter- 
elements, so that the two cells produced contain each the same 
number (twelve) of residual chromosomes. Consequently, 
like the last division in Mammalia, it presents nothing in 
common with the “ Reductionstheilung”’ described by Hert- 
wig, and upon the assumed universality of which so much of 
Weismann’s latest theory of heredity is built. 
39. The process of return to rest of the daughter-cells (sper- 
matids) is in all respects essentially the same as that in the 
previous generation. The distension of the outer spindle-sheath 
(fig. 73) and the formation of intermediate bodies (0. 7.) (cell 
plate) is perhaps more marked, the inner tube (¢.s.) appearing 
consequently more isolated and alone. But there is the same 
detachment of the chromatin from the fibres of the outer sheath 
1 «On the Germinal Blastema of Cartilaginous Fishes,’ ‘Anat. Anz.,’ 
Bad. ix, p. 547. 
