38 JOHNS HOPKINS UNIVERSITY MOEPHOLOGICAL MONOGRAPHS. 



by scattered cells, although it is still sharply defined at a lower level, as is 

 shown in Figs. 8 and 9, 30. The next older embryo, Plate XIV, shows it 

 in about the same condition, degenerated and with no distinct boundary 

 in Figs. 4, 5 and 6, 30, but sharply defined in Figs. 7, 8 and 9, 30. I do 

 not know what becomes of its blastomeres, but as they gradually become 

 unrecognizable, I see no reason for supposing that they persist, and they 

 probably degenerate and disappear. 



The structure which is marked 19 in Plates XII, XIII and XIV 

 becomes the larval organ which is generally known in salpa as the 

 eleoblast. Its successive stages of development in Salpa pinnata are 

 shown at k in Plates XVI, XVII, XXXV, XIX, and in Plate XLI, Figs. 

 2, 3 and 5, k. As these plates show, it grows with the growth of the 

 embryo, and is in the older larvas a prominent organ of considerable size. 



Salensky suggests that it may be a rudimentary tail, and while he 

 does not describe its structure minutely nor present much proof, his 

 view is unquestionably the true one. In Salpa pinnata degenerative 

 changes begin in it very early, at the stage of Plate XVI, and go on as 

 it grows, so that its internal structure is always vague and indefinite, 

 but it is very much less rudimentary in the embryos. of other species, 

 especially those of the cordiformis group, and sections of it, in advanced 

 embryos of these species, show that it is unquestionably the embryonic 

 and degenerated representative of the locomotor tail of the tadpole larvae 

 of other ascidians. In Plate III, Fig. 4, I have drawn an advanced 

 embryo of Salpa hexagona, and in Plate VIII, Fig. 3, I have copied for 

 comparison one of Uljanin's figures of the tailed larva of Doliolum, 

 showing the chorda ch, and the caudal vesicle k, which is formed by its 

 degeneration. 



Comparison of these figures shows clearly that the position and the 

 anatomical relations of the eleoblast, k, of Plate III, Fig. 4, are identical 

 with those of the tail-vesicle k of the Doliolum larva. Fig. 1 of Plate 

 XLIV is a transverse section through the eleoblast of the embryo of 

 Salpa hexagona which is shown in Plate III, Fig. 4. This figure shows 

 that it consists, first, of the outer sheath or cellulose mantle, v ; secondly, 

 of a layer of very thin flat ectoderm cells, a ; third, of a circular tract of 

 the body cavity, 15, filled with blood corpuscles, bl, and migratory follicle 

 cells, 29; and fourth, of a great mass of wedge-shaped or subconical 

 chorda cells, k, radially arranged, with all their protoplasm, and their 

 nuclei, at their outer ends, while the empty bodies of the cells converge 

 towards the center. These conical cells are wedge-shaped in section, but 



