32 The Development of the Thymus 
now an elongated mass of thymic tissue extending upwards behind the 
nodulus thymicus, and fused with it. Its upper pointed extremity curves 
outwards around the hypoglossal nerve. This is the “ thymus super- 
ficialis” (¢ s) of Kastschenko and is regarded by him as being formed 
from the sinus precervicalis. Kastschenko describes this elongated 
portion as being always separate from the rest of the head, being con- 
nected only by connective tissue. In my preparations it is clearly con- 
tinuous with the rest of the anlage, and seems to have formed by growing 
out from it. The presence of a lumen in its lower end favors Kast- 
Atiod.Gu Mitek Bt Text Fic. 4. 
TExT FIGURE 3. Ventral view of thymic anlage; xX 33; pig embryo, 18 mm.; 
ec, ectoderm; 7, lumen; nt, nodulus thymicus; ph, connection to pharynx; 
Sp, Sinus precervicalis. 
TExT FIGURE 4. Ventral view of thymic anlage; X 33; pig embryo, 20 mm.; 
f, area fused with gland of opposite side; 7, lumen; nt, nodulus thymicus; 
th, thoracic segment; ¢s, thymus superficialis. 
schenko’s view, for there is no lumen in the head at 18 mm. in my prepar- 
ations. On the other hand the separation from the head at the 18 mm. 
stage favors the idea that the sinus precervicalis degenerates. J have not 
studied a sufficient number of specimens at this transition stage to 
enable me to decide this point, though I believe the ectoderm takes no 
part in the formation of the thymic anlage. Kastschenko’s results are 
opposed by nearly all other students of this problem, but his work should 
not be discarded before the development of the thymus superficialis in 
the pig has been accurately determined. 
At 27 mm. (Text Figure 5) the thymus is much longer and extends 
