Miscellaneous. 237 



noiform, with a polyhedral transverse section ; they stand with their 

 bases at the surface of the cord ; and all converge towards a longitu- 

 dinal central surface. The whole mass appears as if finely striated ; 

 but the striation does not seem to be due to fibrillar, but only to the 

 edges of the rather thin cells. 



These four cords are united by vascular connective tissue into a 

 thick compact cord, which, lying upon the tube, projects into its 

 lumen. The inner surface of the tube, including the compact cord, 

 is covered by a ciliated epithelium. The four distinct cords lie at the 

 surface of the compact cord something like four cylinders which are 

 enveloped by a larger cylinder touching them. At the Hue of con- 

 tact the ciliated cells are deficient, and the subjacent glandular sub- 

 stance appears freely towards the lumen of the tube. These places 

 are also those towards which the cells converge. Of the four cords, two 

 run into the portion of the tube which extends directly backward, 

 whilst two pass into the spirally convoluted part and follow its con- 

 volutions. In other respects the structure in the hinder part is 

 exactly as in the anterior part. No trace of a neutral fluid is to be 

 found in the gland. 



From this gland the fongue certainly does not originate, as has 

 been concluded from its position, but during the metamorphosis the 

 striated cell-substance disappears. The connective tissue and the 

 epithelial lining of the tubes remain ; the latter separates from the 

 wall, and in part remains tubular, but in part constricts itself into 

 balls. In short, there is produced from it an organ which, both in 

 position and structure, agrees with the thyroid glands of the deve- 

 loped vertebrate. The organ described as the thyroid gland in 

 Pttromi/zon by "VVilhelm Midler (Jenaische Zeitschr, vi. p. 433), I 

 cannot regard as the same, either in structure or position. I have 

 found the true thyroid gland both in P. Planen and P.Jluviatilis ; 

 and it will certainly not be deficient in the other species. In A^n- 

 moccxtes consequently we find for the first time, and hitherto alone 

 among all Vertebrata, the thyroid gland in function during a long 

 period of life and in a high state of development. 



The branchial clefts in Ammocoetes, as is well known, open into 

 the oesophagus — but in Petromyzon into a free tube, closed posteriorly, 

 the bronchus, above which there is an oesophagus which unites the 

 intestinal canal with the buccal cavity. From the mere comparison of 

 Ammocoetes and Petromyzon we cannot see how the new state is 

 produced from the old one. This takes place as follows :— The oeso- 

 phagus is formed in the dorsal median line of the branchial cavity 

 as a solid cord, consisting of round, closely approximated nuclei, only 

 separated by a little interstitial substance ; and into this a cavity 

 penetrates from the front and gradually renders it permeable. At the 

 same time an increase of the blood-vessels commences in the connec- 

 tive tissue which surrounds the branchial cavity and the oesophagus. 

 The vessels finally coalesce, so that both the bronchus and the oeso- 

 phagus lie free in a great l)lood-space, extending from the so-called 

 I)ei-icardium to the point of the head. In this are also situated the 

 branchial arterj", the tongue, and the branchio) themselves. 



