Ree Ball kor 31 
from 11 mm. to 27 mm. . They are graphic reconstructions. Since this 
pouch is nearly all converted into thymus it may be regarded as the 
thymic anlage from a very early stage. 
At 11 mm. (Text Figure 1), the pouch is a hollow epithelial tube di- 
rected from without ventrally and mesially. The lumen (/) is large 
and communicates freely with the pharynx. On the dorso-lateral aspect 
of the pouch is a solid epithelial mass (n ¢) distinctly different in struc- 
ture from the rest of the pouch. This is the nodulus thymicus 
(Kastschenko, 14) and will be referred to by that term. This structure 
has been described by Stieda (26), Prenant* (22), and others as the 
anlage of the carotid gland. 
It was evidently mistaken by Minot * in a 12-mm. pig for the anlage of 
the entire thymus. It may be seen as early as the 8 mm. stage budding 
off from the cranio-lateral aspect of the pouch. Immediately behind the 
nodulus thymicus, but not connected to it at this stage is the inner blind 
extremity of the sinus precervicalis (s p). These become fused at 
12 mm. or 13 mm. 
At 15 mm. (Text Figure 2) the thymic anlage is more elongated. It 
now projects ventrally and medianwards, its free end lying immediately 
caudad to the median thyroid anlage and just craniad to the pericardium. 
Its lumen (1) is still in communication with the pharynx. The sinus 
precervicalis (s p) is drawn out, its lumen being smaller and longer. 
It is now fused to the outer two-thirds of the posterior aspect of the 
nodulus thymicus (n f). 
At 18 mm. (Text Figure 3) the anlage is growing rapidly in a caudal 
direction and just entering the thoracic cavity. It is connected to the 
pharynx by a delicate epithelial cord. There is still a lumen in its caudal 
part. The sinus precervicalis has lost its connection to the nodulus 
thymicus.* The outer part of its lumen has disappeared and it seems 
about to lose its connection with the ectoderm. 
At 20 mm. (Text Figure 4) the thymus extends well into the thoracic 
cavity. Its thoracic segment (th) has increased considerably in size and 
is united to the gland of the opposite side. 
The nodulus thymicus still forms the greater part of the head. The 
anlage has no connection with the pharynx or the epidermis. There is 
2Prenant is said to have since abandoned this idea and accepted Kast- 
schenko’s view. (v. Ebner in K®6lliker’s Gewebelehre des Menschen. Aufl. 
G2 Bdeos 1, S: 5205) 
® Laboratory Text of Embryology, p. 191; also p. 209 and Fig. 124. 
On the opposite side in this specimen, these structures were fused over a 
very small area. 
