152 BULLETIN OF THE 
appears fragmented, and usually has a more or less concentric form 
surrounding the tube (Fig. 50). That face of the nucleus which is 
turned toward the tube is less regular than the other, and usually shows 
a broken and less sharply marked contour than elsewhere. The amount 
of chromatic substance, large in proportion to the somewhat meagre 
supply of protoplasm in the cell, is also noticeable. 
The csophageal-cell with its contained tube traverses the anterior 
chamber; from the mouth opening it first passes through the hypoderm, 
then it lies in a groove on the dorsal surface of the brain (Plate VI. 
Fig. 73), and later is spanned by the dorsal commissure (Fig. 80), di- 
rectly behind which (Fig. 81) it extends a short distance free until it 
reaches and pierces the transverse partition (Plate I. Fig. 8). It is 
enveloped throughout its course by the same peritoneal membrane 
which lines the anterior chamber. The occasional flattened nuclei ‘of 
this membrane may be seen at intervals on the outside of the œsopha- 
geal cell, even where the latter is surmounted by the dorsal commissure 
of the brain. 
Before tracing the further modifications of the oesophageal cell, it is 
interesting to note one or two points of variation in the portion already 
described. Although the tube is commonly of uniform caliber and open 
from end to end, this is not always the case. Figure 50 (Plate III.) 
shows across section of the ossophageal cell 0.1 mm. from the apex of 
the head. Here the tube is of the usual appearance, but a few sections 
farther back not only lumen, but tube as well, has disappeared (Fig. 51). 
Some distance farther posteriad the tube appears again, but as a solid 
cord, which, however, acquires a lumen at a point 0.4 mm. from the 
apex of the head, and from this place on preserves its ordinary character, 
Furthermore, variations in the diameter of the lumen are common. 
The important physiological bearing of these features will be discussed 
subsequently. 
The esophageal tube and cell enter the partition and pass through 
with only a slight expansion in the size of the cell (Plate V. Fig. 63). 
Bürger has figured and called attention (91, p. 643) to the presence of 
a strong dorsal bend of the tube within the partition. This is assuredly 
abnormal, since it is found but rarely. It is entirely wanting in the 
other individual figured by him (Taf. XXXVIII. Fig. 1). When present 
it is probably on, not in, the partition, and is evidently due to the ventral 
flexion of the oesophageal cell and tube resulting from the forcing for- 
ward of the partition in preservation. This wall is in life concave ante- 
riorly, and it will be clear at once, from a glance at Figure 3, that, if at 
