Distribution, Food and reproductive Capacity of some fresh-water Amphipods. H 



the orifices. The eggs are at first without any trace of chorion which, 

 however appears after fecundation. 



Miss Langenbeck observed practically the same method for Micro - 

 deutopus. One female however, whose eggs were just in condition to 

 be extruded, was isolated before it had moulted. After a time this 

 one "sloughed" and the eggs were extruded in the normal way but 

 apparently without copulation with the male. It is stated that these 

 eggs did not segment. 



I was unable to find a female of any species which would deposit 

 eggs without the influence of the male. One female Gammarus 

 limnaeus whose unlaid eggs constituted two conspicuous black masses 

 in the ovaries, was isolated. This one moulted early the following 

 morning, but failed to deposit its eggs during the next twenty -four 

 hours. At the end of this period a breeding male was admitted and 

 immediately clasped the female. Three hours later the marsupial pouch 

 of the latter contained twenty-five eggs which in due time developed 

 normally into as many young amphipods. 



In table IV are given the results of egg counts for the four species 

 In every case animals of various sizes from the smallest mature female 

 to the largest were separately killed, the eggs carefully removed from 

 the brood pouches and counted. 



It can be seen that the number of eggs varies with, (a) the species, 

 (b) the size or age of the individual, (c) with the individual irrespective 

 of size. The average number of eggs produced at one time by an indi- 

 vidual of G. limnaeus is about 25; of G. faciatus, 22; of E. gra- 

 cilis, 45; of Hyalella, 18. In general we find a larger average 

 number produced by the older individuals although the individual va- 

 riation irrespective of size is so great as to prohibit the deduction of 

 any definite ratio. The great variation among individuals of the same 

 species of approximately the same size may be partially accounted for 

 by the loss of eggs due to imperfect fertilization or to accidents to the 

 female. Both cases were observed in the aquaria specimens but it is 

 not known to what extent this takes place in a natural environment. 



5. Period of Incubation. 



In order to determine the amount of time required for the com- 

 plete development of the eggs, it was necessary to capture mated paii's 

 before oviposition had taken place and to isolate them in small dishes 

 supphed with aquatic plants. A maximum-minimum thermometer was 

 suspended near the dishes and readings taken daily. It was observed, 



