220 H. D. Senior. 



self-extension after removal of the egg-membrane (so that it may be 

 fixed in the extended position) the stages have been estimated by 

 the number of somites. Stages have been designated in terms of 

 length of embryo (in a straight line from end to end) from the 

 time that the embryo is approximately straight until the, somewhat 

 arbitrarily selected, jDeriod of hatching. After hatching the age is 

 given as being the sole guide to the stage of development; the 

 length of the embryo, unfortunately, does not convey the required 

 information. 



It is well known that development is relatively accelerated by a 

 high water temperature. The period of development within the egg 

 is actually shorter, however, in warm water, for the embryo is hatched 

 in a progressively immature state in direct proportion as the water- 

 temperature rises. To mention a few examples: My stage of 114 

 hours, 10.5 mm. (just hatched at a low-water temperature^) is much 

 more advanced in development than the stage of 8.7 mm., 63 hours, 

 (hatched about twenty hours in warmer^ water) ; the latter is only 

 slightly more advanced than a stage of 8.3 mm., 107 hours (still in 

 the egg, water-temperature^ low). Direct comparison, except in 

 individual cases, is not easily made between the hatching stages in 

 different water-temperatures because the embryos of any one batch 

 do not hatch simultaneously but continue to hatch over a period last- 

 ing twenty-four hours or more ; the hatching period must, therefore, 

 be in any case somewhat arbitrarily determined. I have selected 

 the stage of 114 hours as the just hatched stage (in preference to 

 earlier "just-hatched" stages) because at all stages prior to this the 

 length of the embryo, from the time it is capable of self-extension, 

 accurately indicates the period of development. 



Shad is anadromous; the egg, demersal and non-adhesive, is con- 

 venient for study on account of its transparency and because it is 

 easily removed from its roomy capsule (diameter of egg proper is 

 about 2 mm. capsule slightly under 4 mm.). Details of the spawn- 

 ing-habits and of the methods of rearing eggs and larva? are given in 

 "A Manual of Fish-Culture," published by the IT. S. Fish Commis- 

 sion, revised edition, Washington, 1900. 



'Average temperature 63° F. 

 •Average temperature 70° F, 



