TRANSACTIONS OF SECTION D. HILL 
formed. Within the cytoplasmic envelopes thus produced the chromatin-grains 
increased in number. Organisms of the degree of structural complexity of a 
true cell arose finally by concentration of the chromatin-grains (chromidiz) into 
a compact organised mass, the nucleus proper. 
II. As regards the origin of the earliest living beings, it is only possible to 
frame vague speculations, in the present state of our knowledge concerning the 
chemistry of the protein-compounds on the one hand, and the metabolism and 
modes of life of the simplest living things on the other. Whether life originated 
on the earth itself, as biologists have generally supposed, or was brought in 
some way to the earth from infinite space, as some physicists have suggested, 
its first origin involves a synthesis of protein-substances in Nature by some 
process as yet totally unknown. When the earliest form of living being had 
come into existence, it is very difficult to understand how it could have main- 
tained its life and what it could have fed upon, especially if it were originally 
wafted from space on to a sterile and barren earth. For light on these problems 
we must look to the future advance of knowledge, and especially of chemical 
science. 
The following Papers were then read :— 
1. The Development of the Thymus, Epithelial Bodies, and Thyrotd in 
the Vulpine Phalanger (Trichosurus vulpecula). By Miss H. A. 
Fraser, B.Sc., and Professor J. P. Hrun. 
Though the development of these structures has been studied in most orders 
of Mammals, nothing hitherto has been known of their origin in the Marsupialia. 
The writers have had at their disposal for investigation a relatively abundant 
material of the diprotodont marsupial 7’richosurus, and are now in a position 
to offer a brief summary of their results. 
fn the adult phalanger, the thyroid is situated just caudally to the larynx, 
and consists of two lateral lobes connected by a median bridge. The thymus is 
remarkable in that it is represented typically by three pairs of glands, viz., a 
large superficial cervical thymus on each side, situated just internally to the 
platysma, and immediately caudal to the submaxillary gland, and two smaller 
thoracic glands, the one cranial to the other, and situated cranially to the 
pericardium in relation to the corresponding common carotid. These thoracic 
glands, as study of their development shows, represent respectively thymus IIT. 
and thymus 1V. They may remain distinct, or they may fuse on one or both sides. 
The epithelial bodies (parathyroids) vary in number, but there are usually 
two on each side, one (epithelial body III.) near the fork of the common carotid, 
and the other (epithelial body IV.) usually not far removed from thymus IV. 
In addition accessory epithelial bodies usually occur in connection with the 
cervical thymus. 
As concerns the development of the thymus, our observations show that the 
epithelial basis of the cervical gland is derived mainly at least from the ectoderm 
of the cervical sinus. The second gill-cleft in early stages is well developed, and 
possesses an extensive area of fusion with the second ectodermal groove. As 
development proceeds, however, the cleft rapidly narrows, whilst the ectodermal 
groove deepens inwards to form the so-called ductus branchialis. Both become 
solid, and we thus have formed an elongated slender cord which ends below in a 
bulbous enlargement formed from the coalesced walls of the cervical sinus. This 
enlargement, which extends from the ventral end of the ductus branchialis to 
below the junction of the third cleft with the sinus ectoderm, represents the main 
primordium of the cervical thymus, which is therefore in greater part at least 
of ectodermal origin, but the participation of the ventral end of the second cleft 
in its formation cannot wholly be excluded since it is impossible to determine 
with certainty the limits between the entoderm of the cleft and the ectoderm of 
the ductus branchialis. 
The third gill-cleft loses its connection with the pharynx relatively early, 
whilst its connection with the ectoderm of the sinus becomes reduced to a thin 
cord which eventually disappears. The dorsal part of the remainder of the cleft 
retains its lumen, the ventral part on the contrary becomes solid. The cranial 
