On many points my investigations are not yet finished, but I have already made out a number of facts which I venture to believe will add to our knowledge of vertebrate embryology; and since it is probable that some time will elapse before I am able to give a complete account of my investigations, I have thought it worth while preparing a preliminary paper in which I have briefly, but I hope in an intelligible manner, described some of the more interesting points in the development of the Elasmobranchii. The first-named species (_Mustelus_ sp.?) was alone used for the early stages, for the later ones I have also employed the other species, whose eggs I have had; but as far as I have seen at present, the differences between the various species in early embryonic life are of no importance.
Without further preface I will pass on to my investigations.
In the eggs of all the species of Dog-fishes which I have examined the yolk lies nearest that end of the quadrilateral shell which has the shortest pair of strings for attachment. This is probably due to the shape of the cavity of the shell, and is certainly not due to the presence of any structures similar to chalazæ.
The yolk is not enclosed in any membrane comparable to the vitelline membrane of Birds, but lies freely in a viscid albumen which fills up the egg-capsule. It possesses considerable consistency, so that it can be removed into a basin, in spite of the absence of a vitelline membrane, without falling to pieces. This consistency is not merely a property of the yolk-sphere as a whole, but is shared by every individual part of it.
With the exception of some finely granular matter around the blastoderm, the yolk consists of rather small, elliptical, highly refracting bodies, whose shape is very characteristic and renders them easily recognizable. A number of striæ like those of muscle are generally visible on most of the spherules, which give them the appearance of being in the act of breaking up into a series of discs; but whether these striæ are normal, or produced by the action of water I have not determined.
The blastoderm is always situated, immediately after impregnation, near the pole of the yolk which lies close to the end of the egg-capsule. Its position varies a little in the different species and is not quite constant in different eggs of the same species. But this general situation is quite invariable. It is of about the same proportional size as the blastoderm of a bird.