. The embryology and metamorphosis of the Macroura. Crustacea; Embryology -- Crustacea. MEMOIRS OF THE NATIONAL ACADEMY OK SCI ENCKS. 397 In insects it appears that a clioriou is always present in ovarian eggs, while, on the other hand, arachnids possess a vitelline membrane and the eggshell is secreted in tlie oviduct. .There is no simple rule to express the appearance of egg membranes in a diversified group like the Arthropods, and, considering that these structures are purely secondary to the cell and expensive products however formed, this is what we should expect. Their function is chiefl
. The embryology and metamorphosis of the Macroura. Crustacea; Embryology -- Crustacea. MEMOIRS OF THE NATIONAL ACADEMY OK SCI ENCKS. 397 In insects it appears that a clioriou is always present in ovarian eggs, while, on the other hand, arachnids possess a vitelline membrane and the eggshell is secreted in tlie oviduct. .There is no simple rule to express the appearance of egg membranes in a diversified group like the Arthropods, and, considering that these structures are purely secondary to the cell and expensive products however formed, this is what we should expect. Their function is chiefly pro- tective, and where a choriou is present in the ovary a yolk membrane is not developed, but the latter is present, as in spiders, when the shell is a later product. Ercll (15) describes three egg- membranes for the lobster, but it is clear, as Mayer lias already shown, that the inner, delicate membrane which has been described for the decapod egg, is a secretion product of the blastoderm. III. SEGMENTATION OF THE EGG OF ALPHEUS MINUS. In the small green Alpheus of the southern coast we have a peculiar modification of the usual process of segmentation, which seems to be anomalous. The fertile egg- is pervaded wi'th a remarkably fine reticulum, which incloses \lk spherules of minute and uniform size. The nucleus is central, or nearly so,* and consists of an ill-defined mass of protoplasm, in which a fluechromatiu network is suspended. In the next phase (I'l. xxvr, Fig. 14) the nucleus is elongated and about to divide. Division appears to bo direct and irregular. At a somewhat later stage the phenomena of the most interest occur (Figs. 12,13). Each product of the first nucleus has developed a swarm of nuclear bodies (S. S.), which seem to arise by fragmen- tation. These bodies take the form of spherical nuclei in clear masses of protoplasm. The yolk frequently has a tendency to segment about the nuclear masses, in the same way that it divides about a single nucleus to form a
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Keywords: ., bookauthor, bookcentury1800, bookdecade1890, booksubjectcrustacea
