6 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



and many otlier flslies, is a nigbt spawner. Although development of 

 the ova proceeds in all parts of the ovary at once, the process is at 

 first more rapid in the posterior (external) end, which consequently 

 becomes spent while the ova production is still active at the anterior 

 (internal) end. The transparent eggs, or more correctly " egg mother- 

 cells," now rupture the follicular membranes which envelop them and 

 are dehisced into the ovarian cavity, where they lie among the ova- 

 bearing laminae bathed in a small quantity of serous fluid. They are 

 slightly smaller and more irregular in shape than the fertilized ova, 

 owing to the somewhat flaccid, wrinkled state of the egg membrane 

 and the mutual pressure to which they are subjected. When first 

 dehisced into the ovarian cavity they are rather opaque, owing to the 

 presence of numerous minute spherules in and beneath the proto- 

 plasmic layer which envelops the fluid yolk. As these spherules are 

 gradually absorbed, the egg mother-cells become more and more trans- 

 parent; but the last granules do not finally disappear until the egg is 

 extruded from the oviduct and feels the stimulus of contact with sea 

 water. Under these circumstances the least opaque eggs immediately 

 assume a crystalline transparency; the more opaque and less mature 

 ones clear up more slowly, but usually as completely, and the process 

 is probably facilitated in this, as in the case of the Spanish mackerel, 

 by the presence of spermatozoa. Immediately upon contact with the 

 sea water, whether or not spermatozoa be present, the egg mother-cell 

 absorbs water and assumes a perfectly spherical shape, the egg mem- 

 brane becomes tense, while the processes attending maturation ensue. 

 The egg now resembles the well-known pelagic ova of the Spanish 

 mackerel, sea bass, and other fishes. It is at first perfectly spherical 

 and varies considerably in size. The average diameter of a large 

 number of eggs from all sources procurable is 1.25 mm., but even from 

 the same female they have varied from 1.16 to 1.20 mm., and it was 

 also found that all of the eggs produced by a particular female were 

 sometimes larger, sometimes smaller, than the average. Thus, all the 

 eggs of the two lots studied at Woods Hole which were measured 

 averaged 1.15 mm., and the individual eggs varied but slightly from 

 this average. On the other hand, all of the Casco Bay females pro- 

 duced eggs of larger size, at the same time presenting a greater range 

 of variation. The horny, transparent, vitelline membrane which sur- 

 rounds the egg is of uniform thinness except at a small circular area 

 which, perhaps owing to its greater weight, always gravitates to the 

 lowermost pole of the floating egg. This is a micropylar thickening, 

 a crater-like elevation projecting inward in contact with the vitellus. 

 A funnel shaped depression exists in its outer surface and extends 

 about half way through, from which point it is continued to the inner 

 surface by an exceedingly narrow and perfectly straight canal, the 

 micropyle, through which the spermatozoan finds its way to fertilize 

 the ovum. Exceedingly delicate canals perforate the whole region of 

 the micropylar thickening and doubtless attbrda means of interchange 

 of water bearing the respiratory gases. Similar but much smaller and 



