There is one peculiarity in the formation of the mesoblast which I wish to call attention to, _i.e._ its formation as two lateral masses, one on each side of the middle line, but not continuous across this line (vide figs. 6_a_ and 6_b_, and 7_a_ and 7_b_). Whether this remarkable condition is the most primitive, _i.e._ whether, when in the stage before this the mesoblast is first formed, it is only on each side of the middle line that the differentiation of the lower layer cells into hypoblast and mesoblast takes place, I do not certainly know, but it is undoubtedly a very early condition of the mesoblast. The condition of the mesoblast as two plates, one on each side of the neural canal, is precisely similar to its embryonic condition in many of the Vermes, _e.g. Euaxes_ and _Lumbricus_. In these there are two plates of mesoblast, one on each side of the nervous cord, which are known as the _Germinal streaks_ (Keimstreifen) (vide Kowalevsky "Würmern u. Arthropoden"; _Mém. de l'Acad. Imp. St Petersbourg_, 1871). From longitudinal sections I have found that the segmentation cavity has ceased by this stage to have any distinct existence, but that the whole space between the epiblast and the yolk is filled up with a mass of elongated cells, which probably are solely concerned in the formation of the vascular system. The thickened posterior edge of the blastoderm is still visible. At the embryonic end of the blastoderm, as I pointed out in an earlier stage, the epiblast and the lower layer cells are perfectly continuous. Where they join the epiblast, the _lower layer cells_ become distinctly divided, and this division commenced even in the earlier stage, into two layers; a lower one, more directly continuous with the epiblast, consisting of cells somewhat resembling the epiblast-cells, and an upper one of more flattened cells (Pl. 3, fig. 4, _m_). The first of these forms the hypoblast, and the latter the mesoblast. They are indicated by _hy_ and _m_ in the figures. The hypoblast, as I said before, remains continuous with the whole of the rest of lower layer cells of the blastoderm (vide fig. 7_b_). This division into hypoblast and mesoblast commences at the earlier stage, but becomes much more marked during this one. In describing the formation of the hypoblast and mesoblast in this way I have assumed that they are formed out of the large mass of lower layer cells which underlie the epiblast at the embryonic end of the blastoderm. But there is another and, in some ways, rather a tempting view, viz. to suppose that the epiblast, where it becomes continuous with the hypoblast, in reality becomes involuted, and that from this involuted epiblast are formed the whole mesoblast and hypoblast.