146 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. [iSbo. 



Ope is wantiug. lu tlie ova of the Ganoids, Amia and Lepidosteus, the 

 zona IS composed, in the first instance, at least, of short, parallel, elastic 

 fibers disposed in a plane vertical to that of the membrane, these fibers 

 being fused at their ends or just below the inner and outer surfaces of 

 the membrane. Sections through the egg membrane of Lepidosteus seem 

 to indicate the same condition of things as in Amia, in fact, Dr. E. L. 

 Mark, of Cambridge, Mass., has kindly shown me drawings which show 

 the fibers of the zona of the former isolated in the same condition as I 

 have been able to separate those forming the egg-membrane of the 

 latter. 



The egg-cases of the oviparous Plagiostomes have a different struct- 

 ure from the zona investing the ova of osseous fishes in that they 

 are fibrous in structure and of a horny consistency, the fibers running 

 in a direction parallel with the surfaces of the case. The ovoviviparous 

 forms, judging from the character of the eggcases of SqualuSj have the 

 walls of the case extremely thin, apparently to facilitate the interchange 

 of the gases needful in foetal respiration during development in the 

 oviducts. 



The skate's eggs have two hollow processes at each end of the pecu- 

 liarly formed case, which are perforated at the sides and which afford 

 passajze for the water in and out of the case needful for the respiration 

 of the developing embryo. The egg-case of Meterodontus or Gyropleii- 

 rodus, a specimen of which has been sent me by Miss Eosa Smith, may 

 be compared to a skate's egg, which is narrower at one end than at the 

 other, but which has been molded into a spiral form during its forma- 

 tion in the oviduct, and which has traces at either end of openings 

 which are apparently homologous with those found in the latter. 



The preceding data render it evident that there is a wide range of 

 variation in the character of the investments which cover the ova of 

 different species of fishes, and it is obvious that, while all of the various 

 forms are for the purpose of giving protection to the ovum, some of them 

 are modified to minister to the respiration of the embryo. In other cases 

 a modification of the membrane such as exists in the adherent ova of 

 Amiurus, may have still another function to subserve, viz, that of en- 

 abling the male parent to aerate the eggs without injury during his ap- 

 parently violent movements over them with his anal and ventral fins. 

 In Amiurus a double membrane invests the egg ; an outer highly 

 elastic one separated from the inner or true zona by a shallow space 

 filled with water, but attached to the latter by irregularly disposed 

 elastic i^illars of the same material as the outer envelope. The outer 

 enveloi)e is also very adhesive, so that the masses of ova in their elastic 

 investments may be repeatedly moved about or shaken up by the old 

 fish without injury or risk of detachment. 



The variability of the egg-membranes of the class Pisces, as restricted 

 by Jordan, is rendered still more striking when the types with filamen- 

 tous appendages, such as Menidia, Belone, Exoca'tus, and Gobius, are 

 considered. No trace of such a structure existing as a covering for the 



