452 STUDIES ON THE EMBRYOLOGY OF THE SIPUNCULIM1 



Fig. 7. Sipunculus embryo seen from the active pole (Hatschek, '83, Taf. II, Fig. 14). The rosette, surrounded 

 by the amniotic cavity of the head, and the mid-dorsal amniotic canal are shown; the further course 

 of this canal toward the vegetative pole and the transition of its margin into the free rim of the serosa 

 are shown in broken lines; the position of the serosa ( = prototroch) is denned. This figure should be 

 compared with Figure 11. 



Fig. 8. An older embryo of Sipunculus than that shown in Figure 7. It is seen from the dorsal side, and shows 

 the amniotic cavities of the head and of the trunk, and the connecting mid-dorsal canal (Hatschek, 

 '83, Taf. Ill, Fig. 31). The cells which underlie these spaces correspond respectively to the apical and 

 the somatic plates and to the connecting mid-dorsal cord of ectoderm in Phascolosoma. The proto- 

 troch or serosa cells at this stage have slipped backward past the somatic plate and closed together at 

 the posterior pole. This figure should be compared with Figure 12. 



Fig. 9. Egg of Phascolosoma vulgare in the 48-cell stage, 6 hours and 40 minutes after fertilization. Drawing 

 of an unstained egg in glycerine, showing the cells of the active pole. The rosette is stippled; the 

 cross-cells are marked with parallel lines; the intermediate cells are shaded very lightly, and the primary 

 prototroch cells upon the margin are shaded more deeply. 



Fig. 10. Longitudinal sagittal section of egg of Phascolosoma gouldii, between 15 and 20 hours old, showing the 

 rosette and intermediate cells of the apical plate and cells of the prototroch. Within are shown endo. 

 derm and mesoderm cells. Compare with Figure 1. 



Fig. 11. Surface view of a young trochophore of P. vulgare, 25.5 hours after fertilization, showing the definitive 

 rosette in the middle of the apical plate, which in turn is surrounded by the cells of the prototroch. The 

 prototroch consists of nineteen cells. The mid-dorsal cord is shown at its junction with the apical plate. 

 Compare with Figure 7. 



Fig. 12. Left-dorsal view of a trochophore of P. vulgare, 25.5 hours after fertilization, showing apical plate, mid- 

 dorsal cord and somatic plate of ectoderm, besides the intervening prototroch. Compare with Figure 8. 



Fig. 13. Section of a trochophore of P. vulgare 39 hours after fertilization. The plane of section is approxi- 

 mately sagittal. Compare with Figure 5. 



Fig. 14. Ventral view of a trochophore of P. vulgare, about 45 hours old, to show the prototroch. The adoral 

 cilia which cover it are represented only along the margin. The postoral circlet of cilia appears behind 

 the prototroch, and is separated from it by a distinct interval. The apical region, with rosette, eye- 

 spots, and sensory flagella, and the rapidly growing trunk are shown. A distinct cuticula is already to 

 be seen in the trunk region beneath the zona radiata. Drawn from a living specimen, with details 

 added from a preparation of a specimen of the same age. Compare with Figure 5. 



Fig. 15. Parasagittal section of a young larva of P. gouldii, 57 hours old, just previous to the casting off of the 

 zona radiata, under which a continuous cuticula has already been secreted. The dorsal and ventral 

 retractors of the right side of the body are shown in a state of partial contraction. Yolk-granules are 

 seen passing out of the prototroch into the clceom, where the entire substance of the prototroch 

 is soon to be engulfed. 



Fig. 16. Parasagittal section of a larva of P. vulgare, 51 hours old but more advanced in development than the 

 larva represented in Figure 15. Yolk-granules of different sizes are seen in the ccelomic fluid, in 

 which they flow freely back and forth at every contraction and elongation of the body, which attend 

 the oft-repeated introversions of the head. The postoral circlet of cilia are still active; a single (left- 

 ventral) retractor muscle, and other features are shown. 



