CHAPTER II. 



THE DEVELOPMENT OF THE SOLITARY SALPA FROM THE EGG. 

 SECTION 1. An Outline of the History of the Egg. 



The history of the eggs of Salpa before they are fertilized is so inti- 

 mately bound up with the history of the process of asexual multiplication, 

 that it is difficult to describe the one without continual reference to the 

 other, and I shall therefore leave the detailed discussion of the origin and 

 homologies of the germ cells for a later chapter, after the process of 

 asexual multiplication has been described. It will, however, be best to 

 speak briefly of the early history of the eggs before we enter upon the 

 description of the process by which the ripe fertilized egg becomes con- 

 verted into a salpa embryo. 



The germ cells are definitely set apart for reproduction at such an 

 early stage, that our account of the embryology of salpa must begin with 

 the embryo of the preceding generation ; for very early in its life, while it 

 is still an embryo, we find in its body cavity a sharply defined mass of 

 cells which the study of older specimens shows to be the germ of the 

 reproductive organs. It is shown at n in Plate XXXV. At the earliest 

 stage in which it can be identified, it lies in the body cavity of the 

 embryo on the middle line of the ventral surface, and it marks the spot 

 where the proliferous stolon is afterwards to be developed. As this latter 

 is gradually formed the germinal mass is folded into it, in a way which 

 is made clear by the successive stages shown in Plate XX, Fig. 6, Fig. 5, 

 Fig. 7; Plate XXXV, n, and Plate XVI, Fig. 5. A series of transverse 

 sections of the proliferous stolon which is shown in longitudinal section 

 in the last figure, is given in Plate XX, Figs. 1, 2 and 3. 



As the stolon lengthens the germinal mass also elongates, as is shown 

 at n, in Figs. 4 and 6 of Plate XLI, so that any transverse section of the 

 former, like those given in Plate XXI, cuts some portion, m, n, of the 

 latter. At first all the cells which enter into the composition of the germ- 

 inal mass are alike, and its structure is homogeneous, as shown in Plate 

 XX, Fig. 6, and Plate XLI, Fig. 7 ; but its peripheral cells soon become 

 arranged in an epithelium, Plate XX, Fig. 2, m; Plate XXI, m, and Plate 



