[9] RETARDING DEVELOPMENT OF SHAD EGGS. 803 



ance of the structure of the yelk took place, or that the epiblastic or 

 hypoblastic coverings of the latter were distorted. 



The epiblastic coverings of the tail, however, showed a tendency to 

 crumple and become distorted. It was also commonly noticed that the 

 epiblast showed a tendency to proliferate or throw out masses of cells 

 in the form of irregular knob-like clusters. These increased rather than 

 diminished in size as development progressed. No other structure of 

 epiblastic origin took part in the tendency to become misshapen. The 

 eyes, nasal pits, and ear capsules were normal in every respect. The 

 heart pulsated more slowly than in embryos hatched in water of the 

 usual temperature. This was probably due to the benumbing effects of 

 the low temperature. 



When deformed embryos were transferred to water of 74° F. they 

 showed no signs of regaining their normal shape, but, on the contrary, 

 the deformity seemed rather to be aggravated as develo])ment pro- 

 ceeded. This was the case also when transferred to water ranging from 

 a temperature of 60° to 64° F. Once established, any deformity in de- 

 velopment seemed irremediable by any further stages which might be 

 necessary to complete the developmental processes undergone in the 



In the light of these researches, taken in their entirety, it would 

 therefore appear that 55° to 53° F. is about the limit to which we can 

 with safety reduce the temperature in which the ova of the shad will 

 undergo their normal development. This temperature would give us, 

 approximately, nine days as the longest period of incubation attainable, 

 time sufficient, added to the four days required for the young to absorb the 

 yelk-sack, or thirteen days in all, to take embryos to be incubated on the 

 route all the way across the Atlantic, or even as far as the Danube or 

 Black Sea. Even this period may be somewhat extended, since it is 

 possible to retard the absorption of the yelk-sack of the young fish by 

 keeping them in water of 60° to 65° F. A temperature of 55° F. would 

 probably not be injurious at this stage. 1 have kept the young in water 

 at 38° F. for half an hour without apparent injury. They had been 

 hatched only a short time before. The cold would benumb them, and 

 they would lie quietly at the bottom of the vessel until restored to ac- 

 tivity as they were warmed up in water of over 70° F., to which they 

 were at once transferred without harm. The muscular masses at the 

 sides of the body were benumbed, as indicated by the quiescent behav- 

 ior of the embryos. Tissue metamorphosis would be hindered by such 

 a fall in the temperature of the water. We saw that the cold caused 

 the pulsations of the heart to diminish in rapidity. This abatement in 

 the activity of the forces concerned in the transformation of the stored 

 protoplasm of the yelk into the structures of the growing embryo would 

 be very marked in consequence of subjecting young shad to a tempera- 

 ture of 55'=> F. By this means, reasoning from what we know of the 

 other phases of development when exposed to like temperatures, the 



