n6 Incubation of the To p- Minnoiv ( Gambusia). [February, 



down over the yelk. In some cases the yelk sac is frequently much 

 absorbed before the outer epiblastic sac begins to collapse. This 

 is the case with Cybium after it leaves the egg, and proves very 

 conclusively that the outer sac is entirely free, laterally and ven- 

 trally, from the inner one containing the yelk. 



There are two principal methods by which the yelk is absorbed; 

 the one where a more or less extensive net-work of vessels is 

 developed over the surface of the yelk, and through which all, or 

 nearly all, of the blood passes to reach the venous end of the 

 heart; in many cases no such net-work is ever developed, as for 

 instance, in the shad, mackerel cod and bonito. To the former 

 class the young top-minnow belongs. Its yelk is orange-colored 

 and imbedded in it superficially are a great number of refringent 

 oil globules of small size. There appears to be a sinus beneath 

 the head, continuous with the segmentation cavity in which the 

 heart is developed. The body of the young fish lies in a groove 

 or furrow on the surface of the yelk. This is the youngest state 

 in which I have seen Gambusia, and explains why I have given 

 the preceding general account of the development of a young 

 fish. The somites or segments of muscle plates had been devel- 

 oped for some time. The heart, brain, intestine and organs of 

 sense were defined. 



The next import mt stage observed, was when the yelk sac was 

 in great part absorbed and the fish nearly ready to hatch, or more 

 properly to leave its follicle and the body of its parent. The ex- 

 traordinary acceleration of development noted in almost every 

 detail of structure, was such as I had never witnessed in any 

 other species of young fish. The bones of the skull, although 

 still cartilaginous, were advanced to a condition not seen in the 

 shad until it has been hatched for three weeks or more. There 

 were intermaxillary elements with teeth ; pharyngeal patches of 

 teeth ; the brain was preity well roofed over by the cartilaginous 

 cranium ; the branchiosteges were developed in cartilage ; the 

 opercles completely covered and concealed the gills, the opercular 

 elements being differentiated ; the gills already bore branchial 

 leaflets ; the neural and haemal arches of the vertebrse were being 

 developed in cartilage ; scales covered the sides and back and were 

 developing in pockets of the dermal epithelium ; in fine, all the fins 

 were already developed except the ventrals with the same num- 

 ber of rays as in the adult, and yet the yelk sac was not nearly 



