796 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [2] 



fectly spberi(.'al, having apparently gained very much in bulk. This 

 gain in size is, however, delusive; it is only the wrinkled egg-membrane 

 which has been distended with water; the vitellus or true germinal and 

 nutritive portion has gained nothing in size. The latter now lies in 

 contact with the lowermost part of the egg-memb:ane when the whole 

 ovum is at rest, and is always more or less depressed from above in the 

 form of an oblate spheroid. After the germ has been developed, which 

 is disooidal in form and placed on the surface of the vitelline si)here, it 

 nsually also occui)ies a lateral position on the vitellus when the ovum 

 is at rest. 



The vitellus rolls about and changes its position inside tbe egg mem- 

 brane as the hitter's position is altered. The vitellus is lieavier than 

 water. A large space filled with fluid now exists between the vitellus 

 and membrane. No adhesive material is found on the outside of the 

 membrane, as in the eggs of the white perch and herring, as nia,\ be 

 readily demonstrated with the microscope, although when tiistextrude<l 

 they are covered with a somewhat sticky ovarian mucus. The ova are 

 heavier than water and rapidly sink to the bottom of the vessels in 

 which they are undergoing development. All of the hatching ai)pa- 

 ratus now used for their incubation in water is operated on the i)rinci- 

 ple of a continuous flow, which keeps the ova constantly in motion. So 

 much for the physical behavior and constitution of the shad-egg, which 

 is necessary for the comprehension of what will be said subsequently. 



It has been the experience of those intrusted with the work of look- 

 ing after the artificial incubation of the epigs of the shad that when the 

 temperature of the water was highest the process was completed soon- 

 est, and when lowest it took a disproportionately longer time. In illus- 

 txation of this fact the subjoined data, supplied by Mr. W. F. Page, are 

 of interest, from the records which were kept at the station on the Po- 

 tomac during the present spring (1881) : 



Lot No. 1. Lot No. 2. 



Time in hatching 148 honrs, 309 hours. 



Avcr.ipe temperature of water 57. 2° F. 64. 5° F. 



Average tomperatme of air 61° F. | 66. 1° F. 



Lot No. 3. 



70 honrs. 



74° F. 

 70. 25° F. 



This series of data shows that with a fall in the temperature of the 

 water down to 57.2° P. it took six days and four hours to complete the 

 development in the egg', with a rise in the temperature of the water 

 to 74° P. the process was complete in a little less than three days. 

 The difference in the times of hatching between lots No. 1 and 3 is 78 

 hours ; the difference in the temperature of the water used is only 

 KJ.So P. Is there a limit to the possibilities of retardation f Experi- 

 ment has shown that there is. The temperature of ice-water, 38° P., 

 was found to be fatal at the morula or germinal disk stage of develop- 

 ment of the shad egg, in the course of exi)eriments made at Havre de 



