178 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



from the parent fish they measure about one-fourteenth of an inch in 

 diameter, are somewhat flattened and irregularly rounded in form; 

 the egg-membrane, a true zona radiata, is much wrinkled and lies in 

 close contact with the contained vitellus. Immediately after impreg- 

 nation this membrane becomes tense, is filled with water which has 

 found its way through the membrane from the outside, and is now per- 

 fectly spherical, 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 con- 

 tact with the lowermost part of the egg-membrane 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 dis- 

 coidal in form and i^laced on the surface of the vitelline sphere, it usu- 

 ally also occui^ies a lateral position on the vitellus when the ovum is at 

 rest. The vitellus rolls about and changes its position inside the egg- 

 membrane as the hitter's position is altered. The vitellus is heavier 

 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 may be 

 readily demonstrated with the microscope, although when first extruded 

 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 apparatus now 

 used for their incubation in water is operated on the principle of a con- 

 tinuous 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 eggs of the shad that when the 

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

 est, when lowest it took a disproportionately longer time. In illustration 

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

 terest from the records which were kept at the station on the Potomac 

 during the present spring (1881): 



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

 water down to 57.2° F. 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° F. 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 



