78 John H. Geroüld, 



Page 



Fourth Cleavage and Sixteen-cell Stage 96 



Fifth Cleavage and Thirty-two-cell Stage 97 



Forty-eight-cell Stage 98 



Forty-eight to Sixty-four Cells 98 



6. Development of the Embryo into the Trochophore (10 — 24 hours) 101 



Apical Plate 



Sinking of its Marginal Cells 



Growth of the Somatic Plate 



Mesoderm 



Entoderm, and the Closure of the Blastopore 



7. Development of the Trochophore (24 — 48 hours) 



Form 



Phototropism 



Apical Plate 



Circlets of Cilia 



Prototroch Cells 



Nervous System 



Mesoderm and Muscles 



Stomadaeum and Proctodaeum 



8. Transformation into the Larva (48 — 60 hours) 



Changes of Form 



Shedding of Yolk Membrane 



Formation of Cuticula 



Neuromuscular Structures 



Muscular Activity 



Establishment of the Coelom 



Dissolution of «the Prototroch 



Overgrowth 



9. Development of the Larva 



Third and Fourth Days 



Fifth and Sixth Days 



Second Week 



Third Week and Later 



Note on the Post-larval Development of Pli. gouldii . 



10. Historical Eeview 



11. Comparisons and Conclusions 



Comparison with Sijmnculus 



Comparison with Chaetopods 



Comparison with Ecbiurids 



Comparison with Molluscs 



Comparison with the Vermidea 



Conclusions 



12. Summary 



13. Appendix 



A. Generic characters of Slpimculus and of Phascolosoma . 



B. Methods 



14. Literature 



