W. K. BEOOKS ON THE GENUS SALPA. 



63 



into the segmentation cavity or body cavity. In Salpa these latter 

 become arranged in an epithelium or continuous layer, and in cut J, I 

 have represented them as beginning to assume this structure by settling 

 down over the inner ends of the blastoderm cells. 



The embryo of Pyrosoma is a disk-shaped blastoderm on a surface of 

 food-yolk ; but so far as the migratory follicle cells are concerned, I regard 

 it as comparable to cuts I and J ; and if we imagine the gastrula in cut 

 J converted into a disk-shaped embryo on the surface of a big yolk, we 

 shall have something like the Pyrosoma embryo, so far at least as the 

 follicle cells are concerned. 



CUT I. 



CUT J. 



We do not know whether the migratory habit of the follicle cells and 

 their peculiar share in the development of many Tunicates were or were 

 not acquired before the embryos departed from the primitive type which 

 is shown in these diagrams; but however this may be, we know that 

 they were acquired by embryos which were, at least, modifications of this 

 type, and we may therefore refer them back to it in imagination and 

 picture them to ourselves as added on to it. 



Between the Pyrosoma embryo and Salpa embryo there is a gap 

 which may be filled, in imagination, in this way. Suppose the follicle 

 cells, which, in cut J, lie in the body cavity of the gastrula, to become 

 arranged over the inner ends of the cells of the ectoderm and endoderm, 

 so as to form a continuous epithelium of elongated cells, as shown in cut 

 K, where 3 is the ectoderm, 9 the endoderm, 15 the body cavity, and 7 

 and 8 the two layers of follicle cells ; and suppose also that besides these 

 follicle cells there are others, in the substance of the endoderm cells, and 

 others in the digestive cavity, and a few outside the embryo in the 

 vicinity of the blastopore. In Plate X, Fig. 8, the follicle cells, which 

 are colored blue, have very nearly the arrangement that they have in 



