ALIMENTARY CANAL. 763 



Stieda (No. 569) has recently verified Kolliker's statements. He finds 

 that in the Pig and the Sheep the thymus arises as a paired outgrowth from 

 the epithelial remnants of a pair of visceral clefts. Its two lobes may at first 

 be either hollow (Sheep) or solid (Pig), but eventually become solid, and 

 unite in the median line. Stieda and His hold that in the adult gland, the 

 so-called corpuscles of Hassall are the remnants of the embryonic epithelial 

 part of the gland, and that the lymphatic part of it is of mesoblastic origin ; 

 but Kolliker believes the lymphatic cells to be direct products of the 

 embryonic epithelial cells. 



The posterior visceral clefts in the course of their atrophy give rise to 

 various more or less conspicuous bodies of a pseudo-glandular nature, which 

 have been chiefly studied by Remak 1 . 



Swimming bladder and lungs. A swimming bladder is 

 present in all Ganoids and in the vast majority of Teleostei. 

 Its development however is only imperfectly known. 



In the Salmon and Carp it arises, as was first shewn by Von 

 Baer, as an outgrowth of the alimentary tract, shortly in front of 

 the liver. In these forms it is at first placed on the dorsal side 

 and slightly to the right, and grows backwards on the dorsal 

 side of the gut, between the two folds of the mesentery. 



The absence of a pneumatic duct in the Physoclisti would 

 appear to be due to a post-larval atrophy. 



In Lepidosteus the air-bladder appears to arise, as in the 

 Teleostei, as an invagination of the dorsal wall of the oesophagus. 



In advanced embryos of Galeus, Mustelus and Acanthias, Miklucho- 

 Maclay detected a small diverticulum opening on the dorsal side of the 

 oesophagus, which he regards as a rudiment of a swimming bladder. This 

 interpretation must however be regarded as somewhat doubtful. 



The lungs. The lungs originate in a nearly identical way in 

 all the Vertebrate forms in which their development has been 

 observed. They are essentially buds or processes of the ventral 

 wall of the primitive oesophagus. 



At a point immediately behind the region of the visceral 

 clefts the cavity of the alimentary canal becomes compressed 

 laterally, and at the same time constricted in the middle, so that 

 its transverse section (fig. 418 i) is somewhat hourglass-shaped, 

 and shews an upper or dorsal chamber d, joining on to a lower 

 or ventral chamber / by a short narrow neck. 



1 For details on these organs vide Kolliker, Entwicklungsgeschichte, p. 88 1. 



