CHAPTER IX. 

 THE ORIGIN AND MATURATION OF THE EGG OF SALPA. 



SECTION 1. The Embryonic Germ Cells. 



Many names have been proposed by various authors for the mass of 

 cells, Plate XXI, m, n, and Plate XXXIV, m, n, and Plate XLI, Fig. 7, n, 

 which runs along the haemal region of the stolon, and gives rise to the 

 reproductive organs of the chain-salpa; but a technical term does not 

 seem necessary, and I shall call it the germinal mass. 



It makes its appearance very early in the life of the embryo, in the 

 form of a sharply defined, compact, subspherical mass of cells, which at 

 first differ very slightly from the other cells of the germ layers of the 

 body of the embryo, although they are, as we shall see, quite different 

 from the follicle cells, which are so numerous during the embryonic 

 stages. In Salpa pinnata, the species in which I have studied it most 

 thoroughly, it is on the middle line of the haemal side of the body at n in 

 Plate XXXV, and between the placenta y" and the eleoblast k. It marks 

 the point where the proliferous stolon is to be developed, and its relations 

 to the other organs of the embryo will be understood by comparing the 

 longitudinal section in Plate XXXV with the series of horizontal sec- 

 tions in Plate XIX, Figs. 1, 2, 3, 4, 5. These sections need little explana- 

 tion. Figure 1 passes through the oesophagus q, and the intestine p, 

 which, in the solitary form of Salpa pinnata, is inside the gill o. Figure 

 2 passes through the ganglion s. Figure 3 is through the eleoblast k, 

 and the endostyle d, d. Figure 4 shows the endodermal tube d' of the 

 stolon opening into the pharynx c, between the halves of the endostyle, 

 and Fig. 5 shows this tube in cross-section at d', and below it the germ- 

 inal mass. A portion of this section is shown, more highly magnified, in 

 Fig. 10, with the germinal mass marked n. Three successive early 

 stages in the development of the germinal mass are shown, more highly 

 magnified, in Plate XX, Figs. 5, 6 and 7. These are portions of median 

 longitudinal sections, like Plate XXXV, from three embryos at three 

 successive stages. Fig. 6 of Plate XX is the youngest stage which I 



