. Text-book of embryology. Embryology. VIII AETHEOPODA 193 daughter nuclei, which form a blastoderm, at first only on the ventral surface ot the egg; only at a later period do cells come to the surface of the yolk on the dorsal side also. When we descend to the lower groups of Crustacea we find that amongst Phyllopoda the development of the Cladoceran genus Pohjphemus has recently been worked out by Kiihn (1912). In fundamental characters it agrees with that of Huphausia; the egg undergoes total segmentation. A 2-cell stage is followed by a 4-cell stage and this by an 8-cell stage in which the
. Text-book of embryology. Embryology. VIII AETHEOPODA 193 daughter nuclei, which form a blastoderm, at first only on the ventral surface ot the egg; only at a later period do cells come to the surface of the yolk on the dorsal side also. When we descend to the lower groups of Crustacea we find that amongst Phyllopoda the development of the Cladoceran genus Pohjphemus has recently been worked out by Kiihn (1912). In fundamental characters it agrees with that of Huphausia; the egg undergoes total segmentation. A 2-cell stage is followed by a 4-cell stage and this by an 8-cell stage in which there are two tiers of four cells, and in which a segmentation cavity or blastocoele makes its appearance. The four cells nearer the animal pole of the egg are larger and clearer than those nearer the vegetative pole, but the latter contain most of the yolk, and in one of them are embedded the remains of the sister cells of the egg, oocytes, which do not ripen, but serve as nourishment. In the 16-cell stage we get two. Fig. 142. —Stages in the development of tlie egg of Polyphemus pedieulus. (After Ktihn.) A, passage from 18-ceU stage to 30-cell stage, from the side. B, 118-cell stage, from below. C, sagittal section through a stage of between 236 and 462 cells, end, endodermal cells; gen, cells of genital rudiment; mes, mesoderm cells. tiers of eight cells each, since every cell except one divides by a meridional cleavage. This exceptional cell is the one of the four situated in the vegetative half of the egg, which has received the remains of the nutritive cells. It divides, not ineridionally, but into an upper and a lower cell; the lower contains the remains of the nutritive cells, it is the rudiment of the genital organs, and is termed the generative cell; the upper is the endoderm cell, and gives rise to the lining of the mid-gut. At the next period of cleavage these two cells do not divide, but all the other cells divide each into an upper and a lower daughter cell (F
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