BULLETIN OF THE UNITED STATES FISH COMMISSION. 197 



on account of a fine screen which was placed over the outlet from the 

 collector, soon began to feed. In a half hour afterwards, I should think 

 fully ten per cent, of the young fish which had already lost their sacs 

 had begun to feed. The evidence of this was the presence of their crus- 

 tacean food in the intestine, in which it could be readily seen with the 

 naked eye through the transparent walls of the abdomen. It would 

 tend to accumulate just behind the origin of the liver and air-bladder, 

 which marks the origin of the larval stomach. 



These experiments were instituted on the 11th day of June, and daily 

 supplies of Copepoda were afterwards sent to the Arm ory by Dr. Hessel 

 from the carp ponds, where the latter gentleman also found other locali- 

 ties where these crustaceans were still more abundant than those origi- 

 nally visited by me. The mortality amongst the young fish, even in the 

 presence of their natural food, was very great — not less than 75 per 

 cent. ; but it was shown that it would be practicable to furnish the required 

 food, and as I hear since I left Washington, from Colonel McDonald, 

 the young fish continue to feed, growing rapidly and giving every prom- 

 ise of surviving until an advanced condition of growth is reached. Some 

 of these young shad, according to my last advices from the central sta- 

 tion, must now, July 6, be nearly a month old. 



Since the above was written some time has elapsed, and the survivors 

 of the lot of young shad which gave such promise of continuing to grow 

 have either been captured for preservation in alcohol or have escaped 

 from the aquarium in which they were confined. The last surviving 

 specimen of the lot lived to be forty-two days old, when it accidentally 

 escaped into the sewer-pipe before it was possible to recover it. Those 

 in charge at the time inform me that this individual was about one and 

 a quarter inches in length at the time of its escape. This would be 

 about the length it would have reached at the age mentioned above, 

 judging from some specimens which were submitted to me for examina- 

 tion from North Carolina ; these having had the good fortune to survive 

 to the age of three weeks, when they measured 22 millimeters long, or 

 about seven-eighths of an inch. The Armory specimens, I am told by Mr. 

 J. E. Brown, fed quite ravenously upon the living Copepoda, which were 

 supplied to them to the last. 



XI MECHANICAL CONDITIONS AFFECTING: THE DEVELOPMENT OF FISH 



OVA. 



There is a class of facts met with in embryological observations which 

 have a significance which, I think, have not received the attention they 

 deserve. They relate more especially to what may be termed the mech- 

 anism or construction of ova, and to the peculiarities of development which 

 grow out of conditions of construction as necessary results thereof. 



Holoblastic ova, for instance, are very differently conditioned from 

 the mesoblastic. Practically, we can scarcely say of the former type 

 that it ever develops a blastoderm, but rather that the whole of it at 



