THYREOID GLAND IN SQUALUS ACANTHIAS 205 



of sinusoidal venules around the latter (figs. 4, 15). This condi- 

 tion is found in embryos of about 19 mm. in length. 



In an embryo of 20.6 mm. (No. 33) this venous plexus estab- 

 lishes the first apparent communication with the arterial system, 

 through anastomoses which it forms with small twigs derived 

 from the mandibular branch of the first aortic arch. The asso- 

 ciation of the thjTeoid with the arterial system is never very 

 extensive and the arterial blood which the gland receives is small. 



The subsequent changes in the vascular system in its relation 

 to the thyreoid gland are found to be: first, a more intimate 

 association of vascular and parenchymatous elements from 

 stage to stage; second, a marked and progressive increase in the 

 area of the vascular bed locally; third, a very evident tendency of 

 the venules around the thyroid to fuse and by their confluence to 

 form the large 'thyroid sinus' (Ferguson, '11). These changes 

 are clearly portrayed in the series of figures 10 to 15. 



Apparently the blood-vessels grow around the thyreoid gland 

 and spread out in the spaces between its plates without exerting 

 any appreciable mechanical influence on it. The gland is, as it 

 were, passively enveloped by the coalescence of the vessels 

 around it. Although this is true in the early stages, during the 

 latter part of the follicular period the follicles increase in size so 

 as to considerably decrease the relative expanse of the thyreoid 

 sinus. By their growth the lumen of the thyreoid sinus is en- 

 croached upon, one main drainage channel of considerable size 

 being left, and a large number of capillarj^ sinusoids (Minot, '00) 

 are formed (fig. 7). 



The vessels appear to have no causal relationship to the for- 

 mation of the follicles, as the follicles do not begin to appear until 

 after the sinus is well formed. Moreover, as noted above, the fol- 

 licles of the Selachian thyreoid develop in the same manner as 

 those of the man, and in this latter form there is no such relation 

 between the glandular elements and the blood-vessels. 



The endothelium lining the sinusoids forms a more or less com- 

 plete investment for the epithelial portions of the gland. In most 

 places the endothelium is present, but in some parts the epithe- 

 lium appears to be in immediate contact with the blood stream. 



