Or{//in of the Organs (j/Salpa. 133 



r_yngeal pouch. While the vesicles are hollow from the first, 

 they have at first no communication with the cavities of the 

 pharyngeal pouches. The first trace of the gill-slit is a fold 

 or diverticulum in the dorsal wall of the pharyngeal pouch. 

 This elongates and soon unites with the v/all of the peri- 

 thoracic vesicle to form a gill-slit. Soon after these are 

 formed the posterior ends of the bodies of the Salp^e begin 

 to push out to the right and left in such a way that the ellip- 

 tical cross section of the body becomes converted into a wedge, 

 with its narrow edge on the left side of a right-hand Salpa, 

 and on the right side of a left-hand Salpa. The two peri- 

 thoracic vesicles are ditferently affected by this 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 is not. 

 Iluis the left perithoracic vesicle of a right-hand Salpa and 

 the right one of a left-hand Salpa become flattened and elon- 

 gated towards the middle line, while the other remains more 

 nearly circular in section. Their relations to the morpho- 

 logical middle plane are fundamentally identical, but as the 

 middle plane itself gradually moves outwards there is an 

 apparent asymmetry. 



Each perithoracic vesicle now becomes extended towards 

 the middle line, where they unite to form the median atrium 

 or cloaca, to which they contribute equally, although the 

 position of the body is such that sections transverse to the 

 long axis of the stolon might easily be misinterpreted and 

 held to prove that the whole median atrium of a right-hand 

 Salpa arises from the left vesicle alone, and that of a left-hand 

 Salpa from the right one alone. The secondary changes of 

 ]iosition are, however, of such a character that it is impossible 

 to describe them in detail without figures. 



Seeliger's account of the perithoracic structures of Salpa 

 (hmocratica (pp. 18, 48, and 63) serves to show how difficult 

 the study of a simple structure may be made by a slight 

 change of position, for phenomena which can be observed with 

 ease in the straight stolon of Salpa innnata are so obscure in 

 Salpa democratica that all the industry and technical skill 

 which Seeligcr has devoted to this species has had very little 

 outcome. 



His account of the history of the perithoracic system is 

 essentially as follows : — The]jerithoracic tubes, which lie calls 

 the " Seitenstrange," are mesodermal in tlicir origin, and are 

 specialized out of a mass of mesoderm cells which gives rise 

 also to the nerve-tube of the stolon and to the genital rod. 

 The mesoderm passes into the stolon from the body of the 

 embryo in an unspccializcd condition, and gradually becomes 



