Further Studies on Reproduction in Sagitta. 285 



These observations make plain the relation of the two ducts to 

 one another and the activity of the egg in reaching the exterior 

 when ripe. Study of a very complete series of sections of Sagitta 

 from the beginning of post-larval life to maturity seemed to be 

 necessary to settle satisfactorily the origin and development of 

 these ducts. I had no opportunity to secure such material until 

 the summer of 1909, when, through the courtesy of the directors 

 of the marine laboratories at Pt. Erin, Isle of Man, and at Helgo- 

 land, I obtained an abundance of Sagitta bipunctata in various 

 stages. I am especially indebted to Mr. Chadwick, curator at 

 Pt. Erin station, for assistance in collecting and fixing material, 

 as well as for many other courtesies. This material was studied 

 while I was enjoying the privilege of working in the Zoologisches 

 Institut at Wiirzburg, Germany. Later, eggs, embryos, and young 

 Sagittas, 7| days old, were studied at Naples. 



Origin and Development of the Ducts 



Hertwig's well-known figures show the four primary germ cells 

 free in the coelomic cavity of the gastrula, later embryo, and re- 

 cently hatched young Sagitta. In the 10-day old Sagitta he figures 

 the two pairs of primary germ cells attached to the body-wall, 

 and each cell covered by a layer of endothelium. In my 7§-day 

 larvae of Sagitta infiata, I find the germ cells not yet covered by 

 endothelium, although the endothelial cells in some cases appear 

 to be creeping up over them. Figs. 1 and 2, are longitudinal 

 optical sections through the germ cells of whole mounts stained 

 with borax carmine, and Fig. 3 is a cross section through one of 

 the two primary oogonia (7^ days) . My later stages are all Sagitta 

 bipunctata. The youngest, obtained from plankton at Naples, 

 have from 4 to 7 cells in each ovary, and show in a single section 

 (Fig. 4) two or three oogonia lying against the body wall and 

 covered by a layer of endothelium. In ovaries containing from 

 16 to 20 oogonia the antrum has begun to form below the germ 

 cells. Figs. 5 and 6 are sections of the two ovaries of the same indi- 

 vidual, showing the same conditions as in Fig. 4, — a group of oogo- 

 nia covered by a layer of endothelium, which is continuous with the 



