908 FISHES — CYPEINODONTID^. 



aoters which it presents from the developing ova of other vertebrates, as the adult Teleosfr 

 is the from remaining classes of the sub-kingdom to which it belongs. The floor of the cav- 

 ity appears to be formed by the hypoblast or innermost embryonic layer, while its roof is 

 formed by the epiblast or outermost skin layer. Gradually this blastoderm, which ha» 

 been derived by cleavage from the germinal disk, grows over the yelk, no part of its epi- 

 blast layer being in direct contact with the hypoblast below on account of the presence 

 of the intervening film of fluid, except at its rim. The embryo also appears to be in 

 fixed contact with the yelk. The blastoderm grows at about an equal rate all around 

 its margin ; the point where the edges of the blastoderm finally close is almost directly 

 opposite the site where tlie germinal disk first appeared ; the closure at last occurs just 

 behind the tail of the embryo where a little crater-like elevation marks the point at 

 which it disappears. The embryo now lies along a meridian of the blastoderm ; its head 

 at the original germinal pole, its tail at the other. The growth ol the blastoderm over 

 the yelk is greatly facilitated by the film or fluid contained in the segmentation cavity, 

 over which it can glide as it grows without friction. This view seems to me to be the 

 most rational yet proposed in explanation of the method by which the blastoderm grows 

 laterally in all directions down over the yelk. Iq some cases the yelk sac is frequently 

 much absorbed before the outer epiblastic sac begins to collapse. This is the case with 

 Cyhium after it leaves the egg and proves very conclusively that the outer sac is en- 

 tirely free, laterally and ventrally from the inner one containing the yelk. 



"There aie 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 reMn- 

 gent oil globules of small siza. There appears to be a sinus beneath the head, con- 

 tinuous 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 Zygonecies and explains why I have given the preceding gen- 

 eral account of the development of a young fish. The somites or segments of muscle 

 plates had been developed for some time. The heart, brain, intestine and organs of 

 sense were defined. 



" The next important stage observed was when the yelk sac was in great part ab- 

 sorbed and the fish nearly ready to hatch, or more properly to leave its follicle and the 

 body of its parent. The extraordinary 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 inter- 

 maxillary elements with teeth ; pharyngeal patches of teett ; the brain was pretty well 

 roofed over by the cartilaginous cranium ; the branchiosteges were developed in cartil- 

 age; the operoles completely covered and concealed the gills, the opercular elements 

 being differentiated ; the gills already bore branchial leaflets ; the neural and- 

 haemal arches of the vertebra) were developed in cartilage ; there was a lagena attached 

 to the auditory capsule ; scales covered the sides and back and were developed in 

 pockets of the dermal epithelium ; in five, all the fins vrere already developed except tho 

 ventrals with the same number of rays as in the adult. All this, and yet the yelk sac 

 was still not absorbed. I have never seen in any fish embryos of the same age an in- 



