LARVAL DEVELOPMENT IN THE SEA URCHIN 69 



basal region of the egg and blastula, and the mesenchyme the 

 most basal region. 



The transformation of the gastrula into the pluteus begins 

 with change from the radially symmetrical form of the early 

 gastrula to a bilaterally symmetrical form. The basal outline 

 of the gastrula becomes somewhat triangular instead of circular 

 (fig. 3 A, basal aspect) and the apical region appears to shift 

 toward one end of the longitudinal axis and becomes the oral 

 lobe (fig. 3 B, lateral aspect). Meanwhile the apical end of the 

 enteron unites with the body-wall at the point near the apical 

 pole where the mouth develops, and the enteron is marked off 

 into three regions, the oesophagus, the stomach-intestine and the 

 rectum or anal region. Also during this period the skeletal rods 

 appear, the body elongates and the anal arms begin to develop 

 (fig 4, lateral aspect). Further development consists in the 

 continued elongation of oral lobe, the appearance of the short 

 oral arms and the elongation of the anal arms and of the body. 

 The fully developed pluteus (fig. 5 A basal and fig. 5 B, lateral 

 aspect) possesses a very definite antero-posterior axis, the ante- 

 rior end being the broad end bearing oral lobe and arms, with 

 the ciliated band running over the arms and oral lobe and the 

 body margins between them, the posterior end, the opposite 

 tapering end. Since the oral lobe represents the apical, and the 

 anal surface the basal end of the original apico-basal axis, it is 

 evident that this axis is no longer straight but curved, in con- 

 sequence of the shifting of the apical region toward the anterior 

 end. The course of the alimentary tract is in some degree a 

 record of this change. In the analysis of the differential inhi- 

 bitions it is essential to keep in mind this modification of the 

 original apico-basal axial relations, as well as the antero-posterior 

 relations of later stages. 



METHODS OF EXPERIMENT 



The reagents used in this series of experiments were potas- 

 sium cyanide, ethyl alcohol, hydrochloric and acetic acids, so- 

 dium hydrate and ammonium hydrate in various concentra- 

 tions. The eggs or embryos were placed in the solutions in 



