908 FISHES—CYPRINODONTID. 
acters which it presents from the developing ova of other vertebrates, as the adult Teleost 
is the from remaining classes of the sub-kingdom to which it belongs. The floor of the cay- 
ity appears to be formed by the hypoblast or innermost embryonic layer, while its roofis 
formed by the epiblast cr outermost skin layer. Gradually this blastoderm, which has 
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 peint where the edges of the blastoderm finally close is almost directly 
opposite the site where the germinal disk first appeared ; the closure at last occurs just 
behind the tai] 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 of 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. In some cases the yelk sac is frequently 
much absorbed before the cuter 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 en- 
tirely free, laterally and ventrally 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 al], 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 ycung top minnow belongs. Its 
yelk is orange-colored and imbedded in it superficially are a great number of refrin- 
gent oil globules of small size. 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 Zygonectes 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 Cevelopment, 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 teeth; the brain was pretty well 
roofed over by the cartilaginous cranium ; the branchiosteges were developed in cartil- 
age; the opercles completely covered and concealed the gills, the opercular elements 
being differentiated; the gills alreedy bore branchial leaflets; the neural and 
hemal arches of the vertebre 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 were already developed except the 
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- 
