DEVELOPMENT OF THE BLOOD VESSELS IN THE FKOG. 225 



occur as nearly as possible simultaneously with the formation of the 

 mouth by perforation of the stomatodaeal septum. 



The first branchial cleft, between the hyoidean and first branchial 

 arches, extends at this stage very close to the surface, but does not 

 actually open to the exterior. 



The thyroid body arises from the bottom of a slight depression 

 in the floor of the mouth as a slender solid rod of cells, which runs 

 backwards and downwards beneath the floor of the pharynx, and 

 enlarges posteriorly into a club-shaped solid body, reaching almost to 

 the anterior wall of the pericardial cavity, but not quite touching it. 



Immediately behind the last or fourth branchial cleft, the pharynx 

 narrows very suddenly to form the oesophagus, which runs back 

 through the constricted neck. The walls of the oesophagus are 

 thick ; the lumen is very small, and for a short distance is com- 

 pletely absent, reappearing further back as a vertical slit, from 

 the ventral portion of which the lungs arise as a pair of lateral 

 diverticula. This solid condition of the oesophagus we have found 

 to be a constant feature in tadpoles of from 7| mm. to about 10£ mm. 

 in length. The lumen is not re-established until some time after 

 the mouth perforation is completed ; and it is for a time exceedingly 

 narrow. 



A solid oesophagus was first described by Balfour in Elasmo- 

 branchs,* in which it persists for a very considerable period of larval 

 development. Concerning it he says : " The solidification of the 

 oesophagus belongs to a class of embryological phenomena which are 

 curious rather than interesting, and are mainly worth recording 

 from the possibility of their turning out to have some unsuspected 

 morphological bearings." The condition is so striking a feature in 

 tadpoles of the age we have mentioned, that it can hardly have 

 escaped the notice of other investigators. We have, however, failed 

 to find any mention of it in the works we have consulted. As to any 

 possible morphological bearing, we have nothing to suggest. It is 

 perhaps worth pointing out that it is not concerned with the formation 

 of the lungs, for these we have already described as arising at a 

 much earlier stage, when the oesophagus has a considerable lumen. 

 The explanation may possibly prove to be a physiological rather than 

 a morphological one. 



* Balfour, " Elasmobranch Fishes," 1878, pp. 217, 218. 



