24 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



pass, first, through the outer fold, 21, of the embryo sac ; second, through 

 the space between the outer and inner fold, which is part of the body 

 cavity of the chairi-salpa, and is, like this, colored yellow ; third, through 

 the inner fold, 22, of the embryo sac ; fourth, through the brood pouch 

 which is part of the cloaca of the chain-salpa and is colored red ; fifth, 

 through the epithelial capsule, &', which is also colored red, and sixth, 

 through the embryo. 



The embryo sac is formed during the early stages of development, 

 and it becomes complete while the embryo is very small ; and as it does 

 not increase in size, the rapid growth of the embryo soon causes it to dis- 

 tend, and the embryo soon pushes through the small opening, stretching 

 this and forcing its way out into the cavity of the cloaca, as is shown in 

 Plate XLI, Fig. 2, and in Plate XXXV. 



As the epithelial capsule has in the meantime been cast off, the sur- 

 face of the body of the embryo is now directly exposed to the water in 

 the cloaca, and is fastened to its wall only around the neck of the placenta. 

 At the stage which is shown in the last two figures the placenta is still 

 inclosed in the embryo sac, although the embryo itself is free ; but at the 

 stages shown in Plate XLI, Figs. 3 and 5, the placenta also is uncovered, 

 and the embryo sac is no longer recognizable, as its folds have been 

 flattened out and obliterated by the growth of the embryo. 



This brief outline of the history of the foetal appendages is enough 

 to make these structures intelligible in the figures of the various stages 

 of development, and we are now in a position to trace the embryology of 

 salpa from the egg, as our description need not be interrupted by refer- 

 ences to these structures. 



SECTION 3. The Migration of the Follicle. 



The salpa embryo consists of elements of two sorts : those derived 

 from the fertilized egg, colored orange in the plates, and those derived 

 from the follicle, which are, with a few exceptions, colored blue. 



The egg, before fertilization, Plate X, Fig. 1, is inclosed in a capsule 

 of follicle cells, m, which are, in ultimate origin, modified germ cells, as I 

 shall show farther on. Each follicle cell is a cell which might have 

 become an egg, although they are not now eggs. They are not fertilized, 

 and while the part they play in the formation of the embryo is very 

 remarkable and interesting, their function is purely nutritive, and they 

 do not become converted into any of the tissues of the embryo. At first 



