192 MARINE AND FISHERIES 



5 GEORGE v., A. 1915 



Our knowledge of the identity of the first intermediate host of the genus 

 Proteocephalus is confined to Barbieri's paper on P. agonis Barb. Although he 

 did not absolutely prove his hypothesis, he collected sufficient evidence to lead 

 one to feel justified in concluding that Bythotrephes and Leptodora are the forms 

 in which the oncosphere of that species develops into the very young plero- 

 cercoids. 



The Egg. 



Up to the present the egg of the genus Proteocephalus has been described 

 for only a few species, but the descriptions all show that it consists of a six-hooked 

 embryo or oncosphere surrounded by three membranes. The outer or first mem- 

 brane is very variable in shape and size while the other two are constant in mature 

 eggs, that is, in eggs showing three pairs of hooks. The third or innermost mem- 

 brane is difficult to differentiate in whole specimens since it is so thin and so closely 

 applied to the embryo. 



The egg of P. ambloplitis is shown in PI. XX, Fig. 11. It is to be seen that 

 the first membrane varies from a nearly spherical shape to that seen in e, which is 

 rarely found. These extreme variations appear in eggs all procured from a single 

 ripe proglottis, but those shown in Figs. 11 a, c and d are commonest. In fact, 

 apparently all the eggs in most ripe proglottides possess these peculiar dumb-bell 

 shaped outer membranes, thus leading one to consider their structure as char- 

 acteristic of the species. At any rate, such appendages do not appear in the eggs 

 of any other of the several species of Proteocephalus I have examined. On the 

 other hand the rest of the egg is quite typical. As suggested in Fig. lid and shown 

 in the extreme in e, these characteristic swellings of the outer membrane aie not 

 quite in line with the longitudinal axis of the egg. In such eggs as shown in Figs. 

 11 a, c and d, from which living oncospheres can be expressed, the outer membrane 

 varies in length from 55/a to 75/i. The second membrane, however, is more con- 

 stant in diameter, varying only from 24;* to 27/*. The third membrane is not 

 easily seen in the intact egg, but parts of it appear after the oncosphere has been 

 pressed out. The granular layer between the second and third membranes as 

 seen in. optical sections is quite uniform in thickness, about one tenth of the 

 diameter of the second membrane, and is composed of fine granules and* spherical 

 yellowish globules scattered about so as to leave irregular, often circular, clear areas 

 through which one can see the oncosphere. The largest of these small granules 

 are, however, apparently identical with the smallest globules, so that the whole 

 suggests fat droplets of various sizes. 



The oncosphere may be easily pressed out under a glass-cover from the central 

 parts of the egg to either of the expansions of the outer envelope, that is, to the 

 space between the outer and second membranes. There it is seen to move vigor- 

 ously, the hooks acting in a manner very similar to that described by LaRue for 

 P. filaroides. A pressure which is not quite sufficient to cause the oncosphere to 

 escape from the second membrane almost invariably stimulates it to begin its 

 movements in situ. These take place outside of the egg-membranes at the rate 

 of about fifteen per minute. From the lO^n sections of ripe proglottides stained 



