ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 333 



All the vesicles are closed and have no connection with each other, and, as I pointed 

 out in my preliminary paper, the thyroid of Myxine in this respect resembles that 

 of Teleosts. Schaffer has found, exceptionally, some of the alveoli dorsal to the 

 oesophagus, which is contrary to my experience. In my large series of sections the 

 vesicles commence somewhat in advance of the first pair of gills at section 2096 

 (cp. the chart, fig. 3). These anterior vesicles are flattened dorso-ventrally, and are 

 therefore wide from side to side, and although the wall consists of the usual thyroid 

 epithelium, only a very slight cavity and contents were observable in the sections. In 

 fact, the anterior vesicles are difficult to find, and can hardly be important functionally. 

 They are closely attached to the thick tough membrane which here binds together the 

 copulo-copularis and perpendicular muscles and the two chondroidal bars, and they are 

 situated immediately over the superior chondroidal bar. The most posterior vesicles do 

 not extend much behind the origin of the last afferent branchial artery, and not so far 

 back as the posterior border of the last gill. At this point the gut becomes deeper 

 dorso-ventrally, and descends to enter the anterior coelom. 



The form and size of the vesicles vary enormously, although most of them are 

 small and circular or oval. In the specimen figured the large and irregular vesicles 

 were confined to the ventral region of the thyroid area, but this may not apply to all 

 cases. Schaffer does not appear to have realised the true form of the alveoli, having 

 evidently only studied detached sections without reconstructing entire alveoli. 



Admirable preparations illustrating the histology of the thyroid may be obtained by 

 intra-vitam injection of the fixative from the heart. I have used mostly Mann's picro- 

 corrosive-formalin. This travels through the vessels with great rapidity — much faster 

 than any coloured injection medium, and the whole animal is fixed throughout in a 

 fraction of a minute, as can be demonstrated by opening up the body in various places 

 immediately afterwards. I find the best stain for Myxine tissues generally is iron alum 

 hsematoxylin, followed by eosin. 



The wall of the thyroid vesicles consists of a single layer of epithelium, in average 

 cases about 20 m high. This may be quite flat and simple in structure, or the cells 

 may be fairly tall and with well-marked intra-cellular contents. Generally they ex- 

 hibit a well-marked resemblance to the thyroid vesicles of other vertebrates. There 

 is, as in man, no basement membrane sensu stricto. It is true the vesicles are enclosed 

 in a sheath, but this is formed by, and belongs to, the surrounding connective tissues. 

 That it is no part of the vesicle is well seen where two vesicles are in close contact. 

 The supposed basement membrane passes across from one vesicle to the other, leaving 

 the contiguous surfaces bare. Or in those cases where a vesicle is closely opposed to 

 the ventral wall of the gut, it will be noticed that the abutting surface of the vesicle is 

 without a membrane. 



The histology of the thyroid cell varies very greatly according to its condition of 

 activity. The nuclei are well marked and vesicular. There is a distinct nuclear mem- 

 brane, and a reticulum with a somewhat dotted chromatic material and nucleoli. Apart 



TRANS. ROY. SOC. EDIN., VOL. XLIX. PART II. (NO. 3). 43 



