THYMUS AND THYROID BODIES. 
637 
blastodermal cells, that is, of the progeny of the primary blastodermal cells, or nutri- 
tive centres, is employed by the nutrient matter of the remaining secondary or 
proper blastodermal cells. In this way “pabulum” is afforded for two purposes, the 
growth of the blastoderma, and the growth of the embryo itself. 
During the early period of the existence of the blastoderma. before the circulation 
has been established, the product of solution of the elder is at once absorbed by the 
younger cells. During the later periods, the product of solution drops into the inci- 
pient loops of the blood-vessels, and so circulates for purposes of nutrition. This is 
an instance of primary lymphatic absorption, and differs in no essential particular 
from the same process in the animal further advanced. We may consider the blas- 
toderma in fact, during the first period of its circulation, as containing very numerous 
lymphatic ducts, instead of a few, as in the more perfect animal. 
In the blastoderma, the process by which nutrient matter passes into the circula- 
tion, or the act of absorption, as it is usually called, is reduced to its most simple 
form, being contemporaneous and also identical with the formation of the imperfect 
capillary network. In the more advanced animal, when the capillary network is 
consolidated, the product of solution of the textures passes or drops into the inter- 
cellular or textural lacunae, which appear to be the radicles of the lymphatic system ; 
a system which in the adult communicates with the blood-vessels only at a few places 
in the neighbourhood of the trunks of the original blastodermal veins. 
The blastoderma may be considered therefore not only as the first form which the 
being assumes after the commencement of development, and as a basis out of, and 
in which its higher structures are to be raised, but also, as has been already stated, 
the organ of primary digestion ; that is, of the appropriation and elaboration by the 
individual of nutritive matter already prepared, to a certain extent, by another indivi- 
dual or organ. 
All the principal organs and parts of the future being are formed in, and out of, 
portions of the blastoderma. The laminae dorsales, the cerebro-spinal axis, the 
visceral laminae, the intestinal tube, heart and liver, derive their origin from this 
source. Their original relation to this part is soon lost sight of from changes in 
their positions, but principally from the increased development of their original blas- 
tema, and its change into the various textures, and from the various arrangement of 
these textures in the organs. 
There are three organs however which still retain their primitive structure after all 
the other parts of the animal have undergone their complete development, so as 
finally to exhibit no trace of their original simple texture and arrangement. These 
organs are the supra-renal capsules, the thymus and thyroid. 
The structure of each of these three organs is essentially the same : they consist 
of masses of nucleated cells. These cells are grouped around numerous germinal 
spots, arranged throughout the mass, and which may be supposed to act as centres of 
origin and of nutrition, each for its own group. The mass of the organ is supplied 
