80 FISH-CULTURAL ASSOCIATION. 
aninstant. Prof. Leidy took what was supposed to be watomace, 
which was said to give the green color, and proved that it was 
of vegetable origin. Ryder has found that it is a vegetable or- 
ganism which enters the oyster. In regard to their artificial 
culture, Prof. Ryder believes that if the water is kept pure by 
flowing currents over the young, they can be reared. In his ex- 
periments they only survived six days, but his arrangements 
were incomplete. If this can be done we can replenish our oys- 
ter beds at our pleasure. 
Mr. HEwLeTT.—Some oysters that are planted in the fall are 
white. Some are green when we get them, but if they do well 
the color comes right. It depends greatly on what the oyster 
feeds on. 
Mr. Puitiips.—The question of breeding is the most impor- 
tant one. It is now reduced nearly to the point that the fish- 
culturists have reached. Prof. Ryder has distinguished the male 
from the female, has taken the ova and fertilized it and devel- 
oped it until the unfortunate sixth day. He has not yet carried 
them beyond that, but no doubt will accomplish it another 
season. 
The PrRESIDENT.—We would like to hear from Col. McDonald 
on the breeding of shad, and their relation to water tempera- 
tures. 
Cot. McDona.p.—The reasons why fishes return to the waters 
-where they are bred, are not well established. We have had 
many theories on the subject, some claiming that they are guided 
by memory, and others by scent. It is more likely that they are 
governed by temperatures, and the question of the relation of 
temperature to the migration of fishes is so important that sev- 
eral European governments, particularly England and Sweden, 
are engaged in investigating it and its particular relations to the 
migration of the herring. A difference of three degrees is often 
very important and may exclude a fish from our coast. The 
duration of the river life of shad is determined by temperature. | 
The young leave the rivers on the arrival of a certain tempera- 
