192 REPOET OF COMMISSIONER OF FISH AND FISHERIES. 



torial position, which occurs on the eighth day, at 57° F., or 175 t. u., 

 after which it gradually assumes a loop-like or pear-shaped form while 

 traversing the lower hemisphere. This shape becomes the more pro- 

 nounced the more nearly it approaches closure. This thickened blas- 

 todermic ring is the seat of the greatest vital activity in the layer, 

 and Siiiy shock sufficient to cause the death of the egg first manifests 

 its effects in the whitening of the ring and its surrounding tissue. 

 The distinct outline of the fish is first seen when the ring is well 

 down to the equator of the egg. This appearance of the outline of 

 the fish, however, does not indicate that the tender stage is passed, 

 for if is seen that a rupture of the germinal layer is quite fatal and 

 is liable to occur until the egg is completely enveloped and some 

 little time has elapsed to allow for the hardening or toughening of the 

 layer. 



The experiment that seemed to force the above conclusions con- 

 sisted in taking a few eggs at a time and allowing them to fall from 

 different elevations upon the canvas trough-covers, after which they 

 were at once replaced in the water. Death following a severe jar for 

 a given stage was indicated by an almost immediate whitening of the 

 egg, but in the case of a less severe jar this clouding of the substance 

 of the egg took place only after *the lapse of several hours. 



During the entire summer run of 1898 the blastopore closed, with 

 very slight variations, at 225 t. u. When, on examining the eggs, it 

 was found necessary to uncover a new lot, the record was always first 

 consulted to find the age in temperature units, and the uniformity of 

 the record in this respect established the fact that the ring closed at 

 225 t, u. However, when it came to the fall run, with its colder water, 

 it was found that the same phenomenon occurred at 250 t. u., and this 

 number was likewise uniform for the entire fall run ; but as fall-run 

 eggs, with but few exceptions, hatched at as close to 900 t. u. as did 

 the summer-run eggs, it must be concluded either that up to a certain 

 period of its growth the progress of the embryo is more rapid (when 

 measured in temperature units) in warmer temperatures than in colder 

 ones, or else it might be considered a point in favor of the argument 

 that the spring and fall runs are made by distinct and separate vari- 

 eties of fish. The former is probably the case, as the slight variations 

 observed in a long summer-run series seem to point that wa3^ 



In attempting a description of the accompanying sketches of sal- 

 mon embryos one is almost necessarily restricted to terms that are 

 not always scientific. Phenomena that appear to the unaided eye in 

 the entire egg are often quite different from the real biological changes 

 taking place in the egg and which can only be seen by means of sec- 

 tions and a high-power microscope. Thus, when reference is made 

 to the " nucleus," the dark central spot or kernel that is visible to 

 the unaided eye is intended. The true nucleus is microscopic in size 

 and is situated in the upper part of the germ disk, where, after the 



