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Bashford Dean Memorial Volume 



regarded as a derivative of a median trough-like fold, the endostyle, such as is found in the 

 floor of the pharynx in Amphioxus and the ascidians. In all vertebrates in v^hich the 

 ontogenetic development of the thyroid has been studied, it arises in the embryo (tfir., 

 Text-figure 62, p. 388) as an outpocketing from the floor of the pharynx. The distal 

 portion of the outpocketing becomes the thyroid gland. The slender stalk persists for 

 a time either as a hollow tube, the so-called thyroglossal duct, or as a soHd cord; but 

 eventually it degenerates and disappears. Goodey (1910.2) made the remarkable dis- 

 covery, in an adult Chlamydoselachus, of a persistent thyroid duct (my Text-figure 74, 

 v.t.) opening into the pharynx through a perforation in the basihyoid cartilage, and ending 

 bHndly where it comes into contact with the thyroid. This, of course, is not a functional 

 duct; but it is comparable to the "thyroglossal duct" found in the embryos of many 

 vertebrates. The so-called duct is Hned with pharyngeal mucous membrane in which 

 are numerous incompletely developed pharyngeal denticles. 



Text-figure 74. 

 Sagittal section (x 15) through the thyroid 

 gland and persistent thyroglossal duct of 



an adult Chlamydoselachus. 

 h.v., blood vessels; d., denticles; e., enamel organ; fo., 

 follicles; I.t., lumen of tube; v.t., vestigial tube (thyro- 

 glossal duct). 

 After Goodey, 1910.2, Fig. 2. 



Since Goodey's account of the thyroglossal duct of Chlamydoselachus appears to be 

 based on a single specimen, I have thought it worth while to investigate the possible 

 occurrence of such a duct in the four large specimens at my disposal. From each specimen 

 the thyroid was excised together with a large block of surrounding tissues including 

 a portion of the basihyoid cartilage and the lining of the pharynx. The material was 

 partially decalcified, then imbedded in celloidin and cut into serial sagittal sections. 

 In each case the series extended completely through the large foramen in the basihyoid 

 overlying the thyroid. In one case only (specimen No. I) there were two foramina; the 

 anterior foramen is very small. This specimen. No. I, is the only one in which a thyro- 

 glossal duct was found (Text-figure 75, d.), and this duct lies within the posterior and 

 larger foramen. In specimens III and IV, a thyroglossal duct is demonstrably absent. 

 In specimen No. II the material is in such poor condition that neither the presence nor 

 the absence of a duct could be determined. 



In the series of sections from specimen No. I, the lumen of the thyroglossal duct is 

 slightly tortuous, so that the continuity of the duct cannot be demonstrated in any 

 single section. Text-figure 75, representing the thyroglossal duct, is a reconstruction 

 from forty successive sections, each about 20 microns thick, and is slightly diagrammatic. 

 The total thickness of the sections used in this reconstruction is about 800 microns 



