ON THE SUPRA-RENAL, THYMUS, AND THYROID BODIES. 71 



absorbed portion passing back so as almost again to separate 

 the cervical 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 united only by the narrow 

 portion which 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 groups are sepa- 

 rated and connected by a more or less dense highly vascular 

 areolar texture. In the thymus this texture is weak or 

 deficient. 



After this period no great change occurs in the thyroid 

 and thymus 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 ganglions 

 of the vagus. 



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 ovum, 

 retaining throughout their existence the original simple 

 cellular constitution of that portion of the germinal mem- 

 brane. 



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



