INVESTIGATIONS AS TO THE LOCALIZATION OF THE MICROMERE-, THE SKELETON-, AND THE ENTODERM-FORMING MATERIAL IN THE UNFERTILIZED EGG OF ARBACIA PUNCTULATA

Abstract

Animal halves of unfertilized eggs of Arbacia punctulata segmented equally and developed into larvæ of animal type: blastule with enlarged apical tuft. Vegetative halves formed micromeres antipolar to the cut side, and gastrulated, differentiating into ovoid larvæ or plutei. Meridional halves all gastrulated. The type of development was independent of the presence or absence of the egg nucleus (Fig. 1). Unfertilized eggs were divided into approximately equal halves by a section perpendicular to the line nucleus-center of the egg (Fig 2a), the plane of section thus being laid as far away from the nucleus as possible, and at random in relation to the egg-axis. Nucleated and non-nucleated fragments formed micromeres and gastrulated with equal frequency. Some of the fragments differentiated as animal halves (Fig. 2). Atypical small cells, particularly on the cut side, which may be mistaken for micromeres are often formed. The results indicate that the micromere-forming and the entoderm- and the skeleton-forming material in Arbacia is located in the most vegetative part of the unfertilized egg, occupying less than half the volume of the egg. The results of Harnly (1926) on Arbacia and of Tennent, Taylor and Whitaker (1929) on Lytechinus are critically discussed.