296 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



In my account of its history, p. 95, 1 have divested it of these secondary 

 changes and have described it as it would be if there were no secondary 

 changes of position, and we have now to consider its actual history as 

 exhibited by sections. 



As the right and left pharyngeal pouches, Plate V, Fig. 1, 27 and 28, 

 and Plate XV, Figs. 6, 7, 11 and 12, 27 and 28, are formed, the perithoracic 

 vesicles, g and h, are folded inwards by the growth of the ectodermal 

 folds, so that each one of them lies on the dorsal or proximal surface of 

 its corresponding pharyngeal pouch. 



"While the vesicles are hollow from the first, Plate XLIV, Fig. 10, n, 

 they have at first no communication with the cavities of the pharyngeal 

 pouches. The first trace of a gill-slit is a fold in the dorsal wall of the 

 pharyngeal pouch, shown in Plate XXIV, Fig. 1, A, at 27 and at 28, in 

 salpa C". This fold or outgrowth from the pharyngeal pouch elongates 

 and soon unites with the perithoracic vesicle, n, to form a gill-slit, Plate 

 XXV, Figs. 5, 6 and 7, G-G' and H-H 1 , at g and h. It will be seen that 

 the endoderm is the active agent in the formation of the slit, and it will 

 also be seen that the union between the perithoracic vesicle and the 

 pharyngeal pouch takes place much earlier than the union of either 

 of these structures with its fellow. 



Soon after the gill-slits are formed the posterior ends of the bodies of 

 the salpae begin to push out to the right and the left, as Plate XXV, 

 Figs. 6 and 7 show. The effect of this change is to convert the elliptical 

 cross-section of the body into a wedge with its narrow edge on the left 

 side of a right-hand salpa, H-H', and on the right side of a left-hand 

 salpa, G-G'. The two perithoracic vesicles are differently affected by the 

 change, for while the one nearest the pointed end of the wedge is com- 

 pressed in the line of the axis of the stolon, the other one is not. Thus 

 the left perithoracic vesicle of the right-hand salpa, n, of H-H', and the 

 right-hand one, g, of the left-hand salpa, G-G', becomes flattened and 

 elongated towards the middle line, while the other, g, of H-H', and h, of 

 G-G', remains more nearly circular in section. 



Their relations to the morphological middle plane are fundamentally 

 identical, but as the middle plane is itself moving outwards, there is an 

 apparent asymmetry which the figures explain more clearly than words. 

 Each perithoracic vesicle now becomes extended towards the middle line, 

 where they unite to form the median atrium or cloaca. They contribute 

 to it equally, and at the stage shown in Plate XXXIII, Fig. 3, they are 

 shown at g and h at N-N' for a right-hand salpa, and at K-K' for a left- 



