108 



all thyroid glands. Hence the foramina must have been occupied by 

 the original endodermic evagination. 



In other specimens of basihyals of S. canicula which have been 

 sectionised, a very curious tubular structure has been found, but 

 more cannot be said about this until investigations have been made 

 with embryological material. When this has been done, I hope to be 

 able to give a fuller account of the whole subject. 



It is very interesting to note that in Man there are sometimes 

 found structures comparable with those described in Chlamydoselachus 

 and Scyllium. In Man the thyroid gland develops from three evagin- 

 ations from the ventral wall of the pharynx, one medially and two 

 laterally situated. The median one is known as the thyro-glossal 

 duct. It remains open in the embryo for a short time, and a 

 portion of the thyroid gland is developed from its lower end. Usu- 

 ally it closes or becomes filled up with tissue and its upper end 

 persists in the adult as a pit-like depression on the dorso -posterior 

 surface of the tongue, known as the foramen caecum. 



Occasionally the thyro-glossal duct persists, and is found as an 

 abnormality in the adult ^) thus corresponding with the vestigial tubular 

 structure described above occurring in Chlamydoselachus. When this 

 is the case it becomes divided, according to Bland-Sutton 2), by the 

 growth of the hyoid bone, which divides it into an upper part, lingual 

 duct, and a lower portion, thyroid duct. 



Tumours, dermoids and fistulae have been found occurring in con- 

 nection with these persistent ducts. Tumours, associated with the lin- 

 gual duct, occasionally occur in the tongue, in the neighbourhood of the 

 foramen caecum, and resemble the thyroid gland in structure. It is 

 not suggested that they are in any way analogous to the thyroid fol- 

 licles found in the foramen of the basihyal of Scyllium, but they are 

 brought into line with these, in order to show the occurrence of thyroid 

 tissue in both, in situations associated with the original median 

 thyroid duct. 



I am greatly indebted to the late Prof. Beidge for the use of a 

 valuable male specimen of Chlamydoselachus anguineus, in which the 

 vestigial tubular connection with the oral cavity was discovered. I 

 should also like to express my thanks to Prof. Gamble for very 

 kindly reading through the M.S., and for much helpful criticism. 



1) I am indebted to Dr. P. P. Cole of the University Medical 

 Staff, for very kindly calling my attention to these structures in Man, 

 and for referring me to the necessary literature. 



2) Tumours, Innocent and Malignant, London 1906, p. 458. 



