EXPLANATION OF PLATE I. 



Figs. 1, 3, 4, and 5. Surface views of embryos, represented as opaque 

 objects within the vitelline membrane. The nuclei of the ectoderm cells, and 

 the scattered mesenchyme cells beneath, are indicated. 



Fig. 1. Posterior view of forty-four hour embryo, showing the peristome in 

 front of the blastopore and the vaso-peritoneal vesicle within, x 175. 



Fig. 2. Sagittal section of embryo represented in Fig. 1. The vaso-peritoneal 

 vesicle is divided into anterior hydrocele and posterior enterocele. The 

 enteron is growing toward the bottom of the peristome. X 175. 



Fig. 3. Ventral view of embryo showing the peristome as a straight groove 

 across the ventral surface, x 175. 



Fig. 4. Ventral view of an embryo of four days. The four of the primary 

 tentacles first developed and the bud of the posterior mid-ventral first pedicel, 

 are well marked, x 175. 



Fig. 5. Embryo represented in Fig. 4, seen from the right side. X 175. 



Fig. 6. One of the three calcareous perforated anal plates, x 135. 



