W. K. BROOKS ON THE GENUS SALPA. 257 



where the ectoderm cells are the longest, and it afterwards extends 

 downwards. Since this process involves the multiplication of the ecto- 

 derm cells, and since this multiplication and division follows growth, we 

 can understand that the construction of the cellulose mantle which 

 begins on the upper surface of the embryo must, as it extends down- 

 wards, bring about the growth of the ectoderm cells in the lower part of 

 the embryo." 



It will be seen that Salensky's view is that the epithelial capsule 

 (Ectodermkeim) is at first thick and made of cubical cells; that it 

 becomes stretched and its cells flattened on the sides of the body by the 

 distention which is produced by the growth of the internal parts, and 

 that it afterwards becomes thick again to form the true ectoderm. 



My own observations show that at first it is uniformly thick ; that it 

 becomes distended at the sides by the growth of the embryo, and that its 

 cells are then thrown off as the true ectoderha becomes developed under 

 them. 



Salensky's exhaustive researches on so many species are worthy 

 most careful consideration, and I have not approached the subject with 

 any preconception, for I am fully prepared to believe, on sufficient 

 evidence, that the ectoderm of the salpa embryo comes from the tissues 

 of the body of the chain-salpa rather than from the egg. 



I think, however, that my observations furnish two reasons for 

 believing that the epithelial capsule is thrown off as I have stated, and 

 that it takes no part in the formation of the embryo. In the first place 

 it will be seen that the flattened cells which cover the sides of the embryo 

 in the stage shown in Plate XXII, are thrown off in the flattened con- 

 dition, as shown in Plate XVI, Fig. 6, &', and that they never become 

 cubical, but are different, even after detachment, from the true ecto- 

 derm cells. In the second place it will be noticed that after the cellu- 

 lose layer, Plate XLV, Fig. 3, F, has made its appearance, these cells 

 are often to be seen scattered over its outer surface, as shown at B', 

 and not in its substance. 



