no Incubation of the Top- Minnow (Gambusia). [February, 



position than in the female. The most remarkable difference pre- 

 sented by the male as compared with the female, however, is his 

 inconsiderable weight, which is only 160 milligrammes, while that 

 of the gravid female is 1030 milligrammes, or nearly six and one- 

 half times the weight of the male. 



The female, as already stated, is larger than the male, and 

 measures one inch and three-fourths in length. The liver lies for 

 the most part on the left side. The intestine makes one turn upon 

 itself in the fore part of the body cavity and passes back along 

 the floor of the abdomen to the vent. The air-bladder occupies 

 two-fifths of the abdominal cavity, and at its posterior end the 

 wolffian duct traverses it vertically, to be enlarged near its outlet 

 into a fusiform urinary bladder of very much the same form as in 

 many embryo fishes. The ovary is a simple, unpaired organ which 

 lies somewhat to the right and extends from the anterior portion 

 of the body cavity to its hinder end, and serves to fill up its lower 

 moiety when fully developed. The ova, when full grown, are each 

 enveloped in a sac or follicle supplied with blood from a median 

 vascular trunk which divides and subdivides as it traverses the ovary 

 lengthwise in a manner similar to that of the stem to which grapes 

 in the bunch are attached. In this way it happens that each egg 

 or ovum has it own independent supply of blood from the general 

 vascular system of the mother, from which the material for the 

 growth and maturation of the egg is derived, and which afterward 

 becomes specialized into a contrivance by which the life of the 

 developing embryo is maintained while undergoing development 

 in their respective follicles in the ovary or egg-bag. The ova 

 develop along the course of the main vessel and its branches, as 

 may be learned upon examining a hardened specimen, where the 

 very immature ovarian eggs are seen to be involved in a mesh- 

 work of connective fibrous tissue, which serves not only to 

 strengthen the vessels but also afterward enters into the structure 

 of the walls of the ovarian sacs or follicles externally. 



The very immature eggs measure from less than a hundredth 

 of an inch up to a fiftieth, and on up to a twelfth of an inch, when 

 they may be said to be mature. They develop along a nearly 

 median rachis or stalk which extends backward and slightly down- 

 ward, and which gets its blood supply very far forward from the 

 dorsal aorta. The ova, after developing a little way, are each in- 

 closed in a follicle, the Grnefian follicle, ovisac, ovarian capsule, 



