From what has been stated with reference to the development of the mesoblast, it is clear that in Elasmobranchii this layer is derived from the same mass of cells as the hypoblast, and receives none of its elements from the epiblast. In connection with its development, as two independent lateral masses, I may observe, as I have previously done[143], that in this respect it bears a close resemblance to mesoblast in Euaxes, as described by Kowalevsky[144]. This resemblance is of some interest, as bearing on a probable Annelid origin of Vertebrata. Kowalevsky has also shewn[145] that the mesoblast in Ascidians is similarly formed as two independent masses, one on each side of the middle line. Footnote 143: _Quart. Journ. of Microsc. Science_, Oct., 1874. [This Edition, No. V.] Footnote 144: "Embryologische Studien an Würmern u. Arthropoden." _Mémoires de l'Acad. S. Pétersbourg._ Vol. XIV. 1873. Footnote 145: _Archiv für Mikr. Anat._ Vol. VII. It ought, however, to be pointed out that a similar bilateral origin of the mesoblast had been recently met with in Lymnæus by Carl Rabl[146]. A fact which somewhat diminishes the genealogical value of this feature in the mesoblast in Elasmobranchii. Footnote 146: _Jenaische Zeitschrift_, Vol. IX. 1875. A bilateral development of mesoblast, according to Professor Haeckel (_loc. cit._), occurs in some Osseous Fish. Hensen, _Zeit. für Anat. u. Entw._ Vol. 1., has recently described the mesoblast in Mammalia as consisting of independent lateral masses. During the course of this stage the spherules of food-yolk immediately beneath the embryo are used up very rapidly. As a result of this the protoplasmic network, so often spoken of, comes very plainly into view. Considerable areas may sometimes be seen without any yolk-spherule whatever.