636 
MR. J. GOODSIR ON THE SUPRA-RENAL, 
crescentic form, its sides being as yet united to the anterior horns of the thymus, 
which pass along the jugular veins. 
The thyroid now separates more completely from the thymus, by the prolongation 
forwards of the absorption previously mentioned from the anterior angles of the 
triangular portion, so as to separate the thyroid from the anterior horns of the thymus ; 
at the same time the posterior angle of the 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 completely 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 opake and homogeneous, the thymus 
consists of minute granular masses imbedded in a semitransparent matrix. The 
component elements of the texture 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 separated 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 their original simple cellular con- 
stitution of that portion of the germinal membrane. 
I shall now endeavour to explain in how far the observations just detailed appear 
to me to enable us to trace the functional import and anatomical peculiarities of 
these organs. 
During the first stage of the development of the animal ovum, digestion and respi- 
ration — the absorption and preparation of nutriment — are carried on by the blasto- 
derma, a structure consisting of nucleated cells and of vessels. 
The cells, of which the blastoderma consists, are the progeny of that previously 
occupying the germinal spot of the ovum, and are continually reproduced and in- 
creased in numbers by the production of others from the nutritive centres, or 
secondary germinal spots distributed over it. 
Materials for the nutrition of the blastoderma are derived from the subjacent 
yelk. The matter resulting from the solution of a certain number of the secondary 
