CHANGES IN REPRODUCTIVE CELLS OF ELASMOBRANCHS. 297 
44, The whole course of the spermatogenesis may now be 
diagrammatically represented as follows: In Diagram I the 
rings under a represent a succession of resting cells in the first 
spermatogenetic period, while the signs of division m! (24), 
n® (24), &e., stand for the successive divisions by which they 
are produced. The number 24 represents the constant number 
of chromosomes in each. 
The cone under represents the synaptic change, while 
under y are represented (by black dots) the cellular generations 
of the second spermatogenetic period, up to the formation of the 
final spermatozoa. 
a p Y 
we2------ ~—_— 
77 (24) No(2) 73 (24) 1(2) 2 PE 
Ree o + ——o+ —o + =u @—- : Eebee 
Ds NS liste a oe 
ea 
Diagram J, illustrating the course of Elasmobranch spermatogenesis. (a) 
First spermatogenetic period. (6) Synapsis. (y) Second spermato- 
genetic period. 7, (24), &c., number of chromosomes in each division of 
first period, where x represents an indefinite number of previous divi- 
sions. 1 (12), &c., same in second. 
VII. Comparative. 
45. As the majority of the operations performed by living 
protoplasm are inexplicable on any structural arrangement in 
the parts of cells which has hitherto been observed, it is 
obvious that the structural relationships which, if known, would 
render the actions of protoplasm self-explanatory, lie some- 
where below the present range of vision, and it consequently 
follows that the theoretical explanations of this or that property 
which protoplasm exhibits are, at bottom, nothing more nor 
less than hypothetical forecasts of the ultimate structure on 
which this or that manifestation of vitality depends. The 
probability of any forecast being true is proportionate to the 
