CHANGES IN REPRODUCTIVE CELLS OF ELASMOBRANCHS, 275 
On the Structural Changes in the Reproductive 
Cells during the Spermatogenesis of Elasmo- 
branchs. 
(From the Huxley Research Laboratory, R. Coll. Sci. Lond.) 
By 
J. E. Ss. Moore, A.R.C.S. 
With Plates 13—16, and figs. in text. 
“ Plagiostomorum spermatosomata, que magnitudine et peculiari quadam 
forma in primis ad evolutionis studium apta sunt, hac ratione iam plurium 
observationes in se contraxerunt.”—LAVALETTE St. GEORGE. 
1. Durine the development of a metazoan embryo, after the 
differentiation of the generative cells from those of the general 
somatic “anlage,” the reproductive elements pursue a course 
of evolution peculiar tothemselves. Instead of attaining to the 
high specialisation, decay, and final dissolution, characteristic 
of somatic tissues, their variation is of much less amplitude, 
but cyclical, and returns at length to the production of elements 
similar to those in which both series started. 
In animals, the course of such a reproductive cycle appears at 
first sight to be differentiated into two distinct periods of activity, 
the one extending from the earliest embryonic development of 
the generative elements to the commencement of the proper 
spermato- or ovo-genesis, the other beginning with the spermato- 
or ovo-genesis and ending with the formation of the mature 
reproductive cells. 
In reality, however, the transition from the first of these 
periods to the second is much more expressive of changes 
