318 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.39. 



Furthermore, there can be no such regularity in the arrangement 

 of the germinal areas as was found in the Caligidse. But in those 

 eggs which remain in contact with the wall of the tube there seems 

 to be a tendency to have the germinal area on the outer side. Yet 

 even here, there is nothing definite in the orientation of the embryo 

 itself. 



Maturation in the external saclcs. — The eggs change in color with 

 advancing development; at first they are white and opaque but 

 gradually assume the color of the pigment which is to distinguish 

 the nauplius when it finally issues. 



This color varies in the different species so far as observed, and is 

 constant in the same species, and so furnishes a good secondary 

 specific characteristic. In spite of the diminutive size of the parasites 

 this color is plainly visible to the naked eye and stands out promi- 

 nently against the background of the fishes' gills. 



Nordmann states that in the case of Ergasilus sieholdii a thrice 

 repeated experiment showed that the interval from the first appear- 

 ance of the blue color in the eggs to the time when the nauplii issued 

 from the egg sacs was fifty to sixty hours. In E. centrarchidarum 

 and E. versicolor it is slightly longer than this, seventy to seventy- 

 five hours, but it is found to vary greatly with the temperature. 



Accordmgly this period of maturing will probably be found to be 

 shorter during the August breeding season than during that of May 

 or November. As yet there has been no opportunity to verify this 

 by actual observation. 



Hatching of the nauplii. — The whole of the eggs in the external 

 sacks of any female hatch at the same time, so that if several females 

 with ripe eggs can be secured and placed in an aquarium together 

 quite a colony of nauplii will hatch over night. In spite of the fact 

 that most of the nauplii are in contact with the wall of the egg tubes, 

 each one does not break through the latter for itself, but the tube 

 ruptures in one or two places only, and all the nauplii issue through 

 the same openmg. Hence the tubes are only rarely broken away 

 from the mother, but are nearly always left as empty bags after the 

 last nauplius has emerged. 



The embryos then rupture the outer or vitelline membrane of the 

 egg, issuing forth covered only with the delicate inner membrane. 

 This is much more difficult to break than the outer membrane, and 

 the movements of the inclosed embryo, with the consequent unfolding 

 of its appendages, frequently enlarges this umer membrane fully one- 

 half before it finally gives way. Nordmann makes the statement 

 that the cutting off of the egg sacs or the death of the mother does 

 not disturb the development of the young. This is true only within 

 certain limits, and those limits must be understood if the statement 

 is to be accepted. All the mother can do for the eggs after their 



