454. REPORT OF COMMISSIONER OF FISH AND FISHERIES. [28] 
weight of the spleen, then at its smallest, and has become smooth, 
shining, dark-red, tender, and resembling coagulated blood. From that 
time on it shrivels again, and at the beginning of September is smaller 
than it ever was before, weighing now 37/55 to ss5p of the entire weight 
of the body. In this condition it remains till the spawning season, 
and begins to increase shortly before the fish return to the sea. The 
volume of the spleen likewise changes in the male fish, but not in as 
regular a manner as in the female; it decreases and increases several 
times during the summer, until the largest size is reached, in the begin- 
ning of October (as much as 74; of the weight of the body). During 
the spawning season it again decreases in size and weight. 
This swelling is nothing else than a filling up with liquid blood. Under 
the microscope we see a net-work of beams, which, combined, form a sort 
of sponge. The outermost branches of the arteries open (as has been 
proved by injections) into the meshes of this sponge, and from this very 
same sponge the capillary vessels of the veins receive their supply. But 
these meshes may be either nearly empty or filled to repletion, accord- 
ing to the degree of contraction of the muscles of the arteries. This 
blood (as has been shown by comparing the coloring quality of the 
watery extract from the spleen with blood from the heart) is much richer 
in blood-particles than the circulating blood from the heart; the sponge 
acts upon the bleod like an imperfect filter, partly retaining or keeping 
back the blood-particles, and at the same time, acts like a basin in the 
sea, in which the rapid current becomes slower, and drops its heavy accu- 
mulations. The whole mass of blood which is retained may, under differ- 
ent mechanical conditions, again join in the general circulation. As is 
shown by numerous comparisons between the veinous blood from the 
spleen and the blood from the heart, colorless blood-cells are formed in 
this stagnant blood, but, as it seems to me, not in such a degree as to 
thereby exhaust the significance of so remarkable an occurrence. 
Weighty reasons lead me to the opinion that such a temporary retaining 
of one-fourth to one-third of the entire quantity of blood, during certain 
phases of the sexual development, forms an important link in the chain 
of causes by which the great change in the internal economy of the en- 
tire body, described above, is introduced. This is not the place to enter 
more fully upon the discussion of this question. I only mention these 
curious phenomena of the spleen as further proof that the most radical 
changes in the formation of the saimon go on during its stay in fresh 
water. 
STATISTICS RELATIVE TO THE MIGRATIONS OF THE RHINE SALMON. 
The question of age.-—At what age do the salmon, for the first time, 
migrate and spawn? Do they spawn every year or at greater intervals? 
Do they perhaps only spawn once in their life, and die soon after? 
Such and similar questions have often been asked from a scientific 
and from a practical point of view; and I feel it my duty to examine 
