No. 2.] DEVELOPMENT OF MANICINA AREOLATA. 207 
the second pair of mesenteries is thus entirely different from 
that of the first pair. All the subsequent mesenteries are 
formed after the fashion of the second pair. 
The outer wall of the larger intermesenterial chamber in Fig. 
18 is made up of unmistakable endoderm, but the inner or 
cesophageal wall has an ephithelium precisely like that of the 
cesophagus. Both are composed of very slender elongated cells 
with a median thickening, in which lies the nucleus ; the periph- 
eral end is enlarged and flattened, so that by the juxtaposition 
of many such cells a continuous cuticle can be formed. The 
cells in question resemble the well-known “supporting cells ”’ of 
the Hertwigs (9). Besides the supporting cells, glandular cells 
are found. These are slender and full of granules, the latter 
staining very deeply with hzmatoxylin. The epithelium of the 
oesophageal wall of this chamber is, moreover, sharply marked off 
from the rest of the epithelium. It is directly continuous with 
the cesophageal ectoderm round the lip of the cesophagus, and is 
evidently a tract of ectoderm. Any doubt which might cloud 
this point is removed by later stages, such as Figs. 44 and 45, 
where the epithelium, which is claimed as ectoderm, overlaps, at 
its upper limit, the endoderm. It is clear that in this stage, Fig. 
18, the cesophageal ectoderm has been reflected round that por- 
tion of the lip (free edge of cesophagus) which belongs to the 
larger chamber, and has then run up along the outer surface of 
the cesophageal tube, driving the endoderm before it. The 
direction of growth is reversed, but otherwise the ectoderm is 
acting in precisely the way which it chose in forming the first 
pair of filaments. In the smaller chamber of Fig. 18, the 
epithelium forming the cesophageal wall does not differ from the 
rest of the endoderm. Ina later stage, when the third pair of 
mesenteries has appeared, the ectoderm is also reflected round 
the lips of this chamber, and runs up along the cesophageal 
wall, Figs. 36 and 39. 
The reflection of ectoderm, which leads to the condition 
shown in Fig. 18, commences as soon as the excavation of the 
primary (larger) chamber begins, and follows close on the heels 
of the latter process. The object of the reflection of ectoderm, 
as will be shown later, is to provide filaments for the young 
mesenteries before the latter are complete; ze., continuous 
from body wall to cesophagus. Bearing this in mind, it is prob- 
