BULLETIN OF THE UNITED STATES FISH COMMISSION. 141 



ones, and every one is aware of the fact that the eggs of the same birds 

 vary considerably in size, and that such variation does not interfere 

 perceptibly -vrith their capability to develop. Between the egg-mem- 

 brane and the germ or vitellns there is always a more or less well 

 marked space or cavitj" filled with water which has been absorbed from 

 without, as is proved by the fiict that when the ova are at first extruded 

 the egg-membrane, in all the species studied by the writer, is more or 

 less relaxed or even wrinkled, and that it is only after they have been 

 in water for some time in the presence of spermatozoa that the mem- 

 brane will distend so as to render it tense and spherical. Unimpreg- 

 nated eggs which have absorbed water are said to be "water-swollen," 

 and usually only a small percentage of them will become so in the ab- 

 sence of living spermatozoa, mixed with them in the water. Ova which 

 have been impregnated and are water-swollen, that is, have developed 

 the water space around the vitellus, are said to have "risen," which is 

 probably in allusion to the fact that in some cases — minnows and shad — 

 a lot of newly laid ova will by this process acquire several times their 

 original bulk, so that if too large a quantity of freshly spawned eggs be 

 put in a vessel the mass will swell in the presence of water so as to fill 

 the receptacle or even run over its sides, somewhat in the same way as 

 leavened dough would acquire increased bulk in a dough tray by the 

 numerous vesicles of carbonic dioxide which are evolved by fermenta- 

 tion and held in confinement by the tough mass of gluten and starch. 

 The water space may in some cases embrace more than two-thirds of 

 the solid contents of the sphere included by the egg-membrane, but this 

 space is smaller and is never at any time more than the 2^*0 of an inch 

 in vertical thickness in the egg of the mackerel. 



Its fnnction is, doubtless, iu the main, respiratory, as suggested by 

 Eansom, who has actually named it the breathing chamber, and it would 

 seem that there is very strong evidence in favor of such an opinion, in 

 that most fish ova die if the water in which they are hatching is not 

 frequently changed. The circumstance that some fish ova are fixed by 

 filaments or bv an adhesive material while the water moves over them on 

 account of its gravity or in consequence of tidal action, would seem to 

 indicate that these modifications were favorable to their aeration, or 

 perhaps, more properly, their respiration, and exchange of salutary 

 oxygen for hurtful gases through the membranous and fluid coverings 

 of the egg. The habit which the male stickleback appears to have of 

 pumping water through the nest with his mouth, so as to change the 

 water surrounding the eggs in his charge, seems to be similarly signifi- 

 cant. 



This cavity serves another purpose. In it the young fish may de- 

 velop immersed in fluid free from friction or hurtful knocks from with- 

 out, if it is a free swimming egg, like that of the mackerel. Its func- 

 tion, aside from that of respiration, w^ould then appear to be essentially 

 that of an amnion, or " bag of waters," such as is developed from the 



