W. K. BROOKS ON THE GENUS SALPA. 21 



SECTION 2. The Foetal Membranes. 



Each egg gives rise to an embryo which becomes a solitary salpa, 

 while the chain-salpae are produced by budding from the solitary salpa. 



The embryo is developed within the body of a chain-salpa, and its 

 growth begins very soon after the chain-salpa is set free and while it is 

 very small, and it keeps pace with the growth of the chain-salpa, so that 

 a fully grown einbryo is gigantic in comparison with the animal which 

 carries it. Plate I, Fig. 3, is an individual of the chain-form of Salpa 

 pinnata with its embryo, but in other species the embryo is relatively 

 very much larger. Each chain-salpa usually contains only one embryo, 

 as is shown in this figure, but in a few species there are several embryos 

 in each. 



Plate III, Fig. 2, is a side-view, and Fig. 3 a dorsal view of the chain- 

 form of Salpa cordif ormis, showing the embryos, e m, on the right side of 

 the body, in the space between the fifth muscle and the sixth. Plate IV, 

 Fig. 6, is a portion of a chain of the same species, showing, on the right 

 side of the figure, the right sides of three salpas, with five embryos in 

 each, arranged in a row in the space between the fifth muscle and the 

 sixth. 



When there are several embryos they are in successive stages of 

 development, as shown in Plate X, Fig. 10, which is from the chain-form 

 of Salpa hexagona, shown in Plate III, Fig. 1, where the five embryos 

 appear as a row of dots on the right side of the body, in the space between 

 the last muscle and the next to the last. 



The egg before it is fertilized, and the embryo during the early stages 

 of its development, lies in one of the blood-channels of the chain-salpa : the 

 space which is marked y in Plate X, Fig. 10, and in the other figures. 

 The egg is suspended by the fertilizing duct, x, of Plate X, Fig. 10, which 

 is fastened to the wall of the cloaca, c, into which it opens. The sperma- 

 tozoa which are drawn into the pharynx of the chain-salpa with the sea 

 water, are swept past this opening by the contractions of the muscles in 

 swimming, and some of them enter it and, penetrating to the egg, ferti- 

 lize it. 



As the embryo grows it pushes in to the cavity of the cloaca, carrying 

 its wall before it, as is shown in Plate XI, Fig. 3, where the letter y marks 

 the blood-channel, while the c above the figure is in the cavity of the 

 cloaca. The layer of epithelium which is marked 6' is that part of the 

 wall of the cloaca which is pushed in before the embryo, and becoming 



