308 
MR. H. GRAY ON THE DEVELOPMENT OF THE 
of the blood-vessels, which is not improbable, these do not form the main element ot 
the spleen. The function that I have explained to be particular to the spleen, takes 
place in the pulp, and in none of the glands of the blood-vessels are the characteristic 
elements of this found.” That the spleen, however, may be classed with these glands 
is 1 think proved, — 
1st. From the manner of its evolution, which is precisely similar both to the supra- 
renal and thyroid glands. 
2ndly. From its structure, which at an early period almost exactly corresponds 
with that of the other glands in question ; and 
3rdly. From the development of its tissues following precisely the same law as that 
upon which the tissues both of the supra-renal and thyroid glands are formed. 
It may be thought by some that I should have made this communication more 
perfect had I traced out the development of the highest of the ductless glands, the 
thymus. The elaborate investigations, however, of Mr. Simon upon this gland, have 
given all that can be desired upon this point, and the few observations I have myself 
made on the evolution of this organ in the chick accord so exactly with his state- 
ments, that 1 could add nothing but what was confirmatory of his observations. 
Explanation of the Plates. 
PLATE XXI. 
Fig. 1. Represents the rudimentary intestinal canal from the embryo of the chick at 
the seventy-second hour. V. Vitelline sac. I. Intestine. L. Liver. 
S. Stomach. 
Fig. 2. Represents the anterior prolongation of the intestinal canal at the ninetieth 
hour, with the liver and pancreas arising as protrusions from that tube. 
P. Pancreas. 
Fig. 3. The same parts are represented as they are observed at the 114th hour. The 
first trace of the spleen is here shown as a small oval body developed in a 
fold of the intestinal laminae, distinct from the pancreas. Sp. Spleen. 
Fig. 4. Represents the intestinal canal, pancreas and spleen from an embryo chiek 
between the fifth and sixth days. The spleen is now observed to have 
approximated close to the end of the pancreas. 
Fig. 5. Represents the same parts from an embryo chick between the sixth and 
seventh days. The pancreas with its duct are observed occupying the fold 
of the duodenum ; the spleen occupying its distal extremity. 
Fig. 6. In this figure are represented the stomach, duodenum, pancreas and spleen 
from an embryo chick between the seventh and eighth days. 
Fig. 7- The duodenal fold is here represented with the pancreas and its duct con- 
