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 oesophageal 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 imtil 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 oesopha- 

 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 oesophageal 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 oesophageal tube and cell enter the partition and pass through 

 with only a slight expansion in the size of the cell (Plate V. Fig. 63). 

 BUrger 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 



