BULLETIN OF THE UNITED STATES FISH COMMISSION. 137 



questiou seems to be, iiiuler wliat couditious can the greatest number of 

 eggs be obtained I Given a sufficient quantity of these, although the 

 losses in hatching may be as much as 50 to 75 per cent., the number of 

 young which it is possible to add to those hatched out naturally will 

 still be prodigious. 



OVARIES AND OVARIAN EGGS OF THE SPANISH MACKEREL. 



The ovary of this fish is a paired organ composed of two nearly cylin- 

 drical sacks lying in the hinder upper portion of the abdominal cavity ; 

 both taper to blunt conical points anteriorly, and are joined posteriorly 

 into a wide common ovarian duct, which opens just behind the vent. 

 Attached to the walls of the ovarian sacks are a vast number of ovarian 

 leaflets or folds i^laced transversely, and which depend directly into the 

 space within the sacks. In these leaflets the ovarian eggs are devel- 

 oped, each one in a minute sack or follicle of its own, tne walls of which 

 are richly supplied with capillary blood-vessels joined to the blood sys- 

 tem of the parent fish. At first the ova are very small, but as the sea- 

 sou advances they, for the most part, increase in size, in consequence 

 of which the entire ovary increases in bulk. At first, when they begin 

 to grow larger, they are barely distinguishable from the ordinary cells 

 which compose the great proportion of the tissue of the ovary ; they 

 are in fact nothing more than greatly enlarged cells when mature, in 

 which we may distinguish an outer germinal layer or i)ellicle, <//>, Fig. 

 2, covering a store of nutritive material known as the yelk, which is 

 gradually absorbed as development progresses ; besides, they are cov- 

 ered by an egg-membrane, zr^ of extreme thinness, perforated at one 

 point only by a minute pore known as the micropyle, which is shown in 

 two i30sitions in the same &gg in Fig. 1, lying in the center of a circular 

 area which has faint markings running out radially from the micropylar 

 pore in its middle toward its margin. The micropyle is funnel-shaped 

 and the radial markings and area around it seem to disappear almost 

 entirely after impregnation. The egg-membrane may be regarded 

 merely as a protective covering and the micropyle as a passage-way for 

 the male element or spermatozoau to find its way through the egg-mem- 

 brane and to the germ, in order that impregnation may take place, when 

 the development or growth of the embryo fish will commence. The 

 opening also connects the space inside the egg-membrane between the 

 latter and the globular Q,gg or germinal mass with the water outside in 

 which the egg floats ; the space here alluded to does not usually api)ear 

 until immediately after impregnation, in consequence of which the egg- 

 membrane at first lies laxly on the germ within, and in the eggs of some 

 species, as in the shad, it is at first considerably wrinkled. It is only 

 after impregnation that it normally absorbs water through the micropyle 

 and becomes tense and perfectly globular. The history of the formation 

 of the egg-membrane is not very clearly established, but it appears in 



