ON THE SUPKA-KENAL, THYMUS, AMD TJIVKOID BODIES. 71 



absorbed poi-tiou passing back so as almost again to sopai-ate 

 the cen'ical portion of the thymus into two lateral portions. 



As development advances the thyroid becomes more com- 

 pletely separated from the thymus, and the lateral portions of 

 the cervical part of the latter are miited only by the naiTow 

 portion Avhich connects them with the thoracic lobe of the 

 organ. 



At this stage a distinction may be observed, with low 

 magnifying power, in the texture of the two organs. The 

 thyroid is more opaque and homogeneous, the thymus consists 

 of minute granular masses imbedded in a semitransparent 

 matrix. The component elements of the textures of the two 

 organs is however identical — namely, simple nucleated cells 

 grouped around dark points, which I am inclined to regard as 

 centres of nutrition. In the thyroid, these gi'oups are sepa- 

 rated and connected by a more or less dense higldy vascular 

 areolar texture. In the thymus this texture is weak or 

 deficient. 



After this period no great change occurs in the thjToid 

 and tliynms of the sheep ; the anterior extremities of the 

 horns of the thymus on each side presenting two bulbous 

 enlargements near the base of the skull, close to the gaughons 

 of the vagiis. 



Four minute white cords may now be seen passing into 

 the superior, and two into the inferior border of the thyroid. 

 These are the inferior and superior thyroid arteries, branches 

 respectively from the first and second branchial arteries. 



From these observations it would appear that the supra- 

 renal capsules, the thymus, and thyroid, are persistent portions 

 of the membrana intermedia of the germinal area of the oviim, 

 retaining throughout their existence the original simple 

 cellular constitution of that portion of the germinal mem- 

 brane. 



1 shall now endeavour to explain in how far the observa- 



