34 The Development of the Thymus 
enlargements which we may call the head, the mid-cervical segment, and 
the thoracic segment. I will consider each part separately. The thoracic 
segment develops rapidly, spreading out above and in front of the heart. 
The glands of the two sides fuse completely in this region. ‘The lymphoid 
transformation is noticeable at 3.5 cm. and well advanced at 4.5 cm. The 
medulla begins to form at 8 em. The cervico-thoracie cord is at first 
very narrow but soon thickens and joins the cord of the opposite side. 
At full term they form a sharp constriction, 3 mm. to 4 mm. wide and 
5 mm. to 6 mm. long, situated at the superior aperture of the thorax, 
and connecting the mid-cervical and thoracic segments. The histological 
changes take place here later than in the enlargements. 
The mid-cervical segment develops like the thoracic segment but some- 
what more slowly. Budding, lymphoid transformation, and formation 
of the medulla all begin here a little later than in the head and thoracic 
segment. Its caudal end is shghtly in advance of its cranial end. The 
intermediary cord is well marked at 4 cm. It soon becomes very attenu- 
ated, having at 6 cm. in many places a diameter of only 15. Prenant 
suggests that this drawing out of the gland is caused by the rapid growth 
of the neck. Later it increases in size and at full term is noticeable only 
as a very slight constriction between the head and the mid-cervical 
segment. The histological changes are much later here than in other 
parts of the gland. 
The greater part of the head in the early stages is formed by the 
nodulus thymicus. This body grows slowly, attaining a diameter in cross- 
section of .6 mm. at 8 em. Its cross-sectional area at 8-cm. is about one- 
third that of the rest of the head of the thymus. At this stage the nodulus 
thymicus is a rounded body lying on the inner aspect of the head in re- 
lation to the carotid artery. A small area of its outer surface is fused 
with the lymphoid tissue of the thymus. Its histological structure has 
been fully described by Prenant (22). From the earliest stages, it 
consists of cords of epithelial cells separated by blood capillaries. At 
8 cm. the thymus superficialis (IXKastschenko) is a large body lying craniad 
and dorsal to the rest of the head but connected to it at its caudal 
extremity. 
I have no observations on the head of the thymus between 8 cm. and 
full term, having overlooked it in collecting my material. In a full term 
pig (30 em.), the cervical part of the thymus is in two distinct parts. 
The postero-ventral part, representing part of the head, the intermediary 
cord, and the mid-cervical segment, is about 3 em. long and 1 em. wide. 
It extends from the upper border of the thyroid cartilage to the thorax, 
and with the corresponding part of the opposite gland encloses the 
