. The Biological bulletin. Biology; Zoology; Marine biology. 298 SVEN HORSTADIUS Arhacia punctulata. Because of the short time available for this in- vestigation the material examined was scanty, but as the results of the observations are very definite, I think they are worth publishing. They indicate that localization in the Arhacia egg does not differ from that in Paracentrotus. The eggs were cut with a fine glass needle manipulated by hand. In order that the eggs should not slip away during the sectioning they were placed in a little scratch in a piece of celluloid (Horstadius, 1928). The c
. The Biological bulletin. Biology; Zoology; Marine biology. 298 SVEN HORSTADIUS Arhacia punctulata. Because of the short time available for this in- vestigation the material examined was scanty, but as the results of the observations are very definite, I think they are worth publishing. They indicate that localization in the Arhacia egg does not differ from that in Paracentrotus. The eggs were cut with a fine glass needle manipulated by hand. In order that the eggs should not slip away during the sectioning they were placed in a little scratch in a piece of celluloid (Horstadius, 1928). The cut was orientated either in relation to the pronucleus (Fig. 2), or to the micropyle, which was made visible by ink from the squid {Loligo) (Fig. 1).. Fig. 1. Cleavage and differentiation of animal and vegetative halves of the unfertilized egg of Arbacia punctulata. Orientation by means of the micropyle (a, b). The egg nucleus may lie in the animal or the vegetative half (a, b). c. Equal cleavage of the animal half. d. The vegetative half has formed macro- and micromeres, the latter at the original vegetative pole, antipolar to the cut side, where a cytoplasmic lobe has protruded through a slit in the fertilization membrane, e. Differentiation of the animal halves into blastulce with enlarged apical tuft. /, g. The vegetative halves gastrulate and form skeleton, developing to ovoid larvae (g) or plutei (/). The results indicate a segregation of the micromere-, the entoderm-, and the skeleton-forming material in the vegetative part of the egg. II. Isolated Animal and Vegetative Halves, and Meridional Halves Twelves eggs were divided equatorially. At the operation the pronucleus in some cases lay close to the plane of section, in other cases far away from it. In 7 pairs the egg nucleus lay in the animal half, in 5 in the vegetative (Figs. \a, h). The animal and the vegeta-. Please note that these images are extracted from scanned page images that may have been digitally enhanced
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