214 WILSON. [Vo. Il. 
one-third the length of the cesophagus, while it will be remem- 
bered that in this stage (Fig. 18) the reflected ectoderm extends 
nearly the whole length of the cesophagus. Now what has 
taken place in the transition of the stage Fig. 18 into the stage 
Fig. 27? Plainly it is that the mesenteries 3 and 4, as they 
grew down and gradually became complete, carried along with 
them the reflected ectoderm, part of which came to lie along 
the mesenteries as short filaments, while the remainder was 
divided into three portions. Of these, the two lateral, which 
originally extended from 2 to 4 on the one side, and from 1 to 3 
on the other (Fig. 27), were carried all the way down to the lip 
of the oesophagus, while the middle portion &.Z. was not car- 
ried the whole way down. 
A couple of longitudinal sections will further elucidate the 
series of changes which separate Fig. 18 from Fig. 27. The 
sections given in Fig. 25 were made from a larva at about the 
same stage as Fig. 27. The two mesenteries of the first pair 
had long filaments, and the mesenteries of the second pair were 
complete and equally advanced. The section @ is a radial half- 
section through one of the second pair of mesenteries, say 4 in 
Fig. 27. The mesentery is complete, and to its edge clings a 
short filament. The other mesentery of the second pair, with 
its filament, is the exact counterpart of the one figured (in cut- 
ting the larva, radial sections of one mesentery and equally 
true transverse sections of the other were obtained). The pre- 
cisely horizontal plane in which the filament of 0 lies, is prob- 
ably due to the sudden expansion of the larva, on being killed, 
in the direction of the shorter transverse axis of the cesophagus 
(the transverse section would be elongated in a direction at 
right angles to the long axis of Fig. 27). The half-section ¢ in 
Fig. 25 is reversed so as to complete the cesophageal lumen. 
It is the second section to one side of 4, and a is the second or 
third on the other side. Comparing all these figures with Fig. 
27, 6 is through mesentery 4, a is on the far side of 4, and con- 
sequently cuts the reflected ectoderm #.Z.; ¢ is on the near 
side of 4, and the ectoderm is not reflected round the free edge 
of the cesophagus. 
Fig. 24 is a single longitudinal section from a stage slightly 
older than the one just described. On the right it is through 
one of the second pair of filaments, and on the left through an 
