. The embryology and metamorphosis of the Macroura. Crustacea; Embryology -- Crustacea. 434 MEMOIRS OF THE NATIONAL ACADEMY OF that the primary yolk cells are formed iu this way rather than by emigration, and that my failure to detect the actual process is due to the fact that I did not sec- tion exactly the right stage, the egg shown in Figs. 38-45 being a trifle too old. In Homarus the primary yolk cells arise hv delamination, as 1 have already shown in a preliminary paper iu the development of this form (!'.'!, Fig. .">). Sections of this stage show conclusively that the
. The embryology and metamorphosis of the Macroura. Crustacea; Embryology -- Crustacea. 434 MEMOIRS OF THE NATIONAL ACADEMY OF that the primary yolk cells are formed iu this way rather than by emigration, and that my failure to detect the actual process is due to the fact that I did not sec- tion exactly the right stage, the egg shown in Figs. 38-45 being a trifle too old. In Homarus the primary yolk cells arise hv delamination, as 1 have already shown in a preliminary paper iu the development of this form (!'.'!, Fig. .">). Sections of this stage show conclusively that the primary yolk nuclei do not come from any one point on the surface, but that the majority of them may come from a restricted area of the egg. In Fig. 3 about two-thirds of the nuclei present are con- fined to the lower (ventral ?} surface of the egg. They are in various degrees of progress from the surface toward the central parts, which the majority have already reached. The formation of primary yolk cells is followed by the in vagination and ingrowth of certain cells at the surface. The his- tology of the embryo at this phase is given in PI. xxxi, and Fig. (> (of text) is constructed from the entire series of sections to show a" "ie primary yolk nuclei present. The plane of the paper (sup- posing the drawing to represent, a sphere) nearly passes through the point of invagination (in.). In order to test the accuracy of the method, two eggs of this stage were studied («, n and b, u ot Table i), aixl the results show a remarkable agreement. Thus there are exactly thirty-seven primary'yolk nuclei in each egg, and the total difference in the number of embryonic nuclei in the two eggs is only nine. Curves were constructed to show the number aud distribution of yolk nuclei and embryonic nuclei in both eggs, and the two are introduced here because of the striking simi- larity. Figs. 7 and S are constructed from the egg seen in Fig. G(n, n, of Table i). Figs. 10 and '.) r
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