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The Development of Phascolosoma. 



(Studies on the Embryolog-j^ of the Sipunculidae IL) 



By 



John H. Oeroiild, 



Dartmouth College, Hanover, N. H. 



"With plates 4—11 and 4 flgvires in text. 



Table of Contents. 



Page 



1. Introduction 79 



2. Development of the Germ Cells 81 



3. Breeding Season and Egg-laying 83 



Segregation of Ova and Spermatozoa into the Nephridia . . 83 



Ejection of Germ Cells 85 



Experiments showing the Influence of Light on Egg-laying . 85 



4. Maturation and Fertilization of the Egg 86 



Polarity of the Egg 86 



First period : Entrance of the Spermatozoon 86 



Second period: Giving off of the First Polar Body ... 86 

 Third period: Formation of the Second Polar Body and the 



Enlargement of the Sperm Nucleus 88 



Fourth period: Pinal Movements of the Pronuclei and their 



Union 90 



5. Segmentation of the Egg 92 



Method and Nomenclature 92 



First Cleavage and Two-cell Stage 93 



Second Cleavage and Four-cell Stage 94 



Third Cleavage and Eight-cell Stage 95 



