THE DEVELOPMENT OF ALCYONIUM DIG [TATUM. 83- 
stomodoeal invagination the free edge of the mesentery 
becomes thickened by a proliferation of the endoderm cells in 
that region. These cells are longer than they are wide,, 
whereas those on the rest of the mesentery are short and 
broad. After the mouth and stomodseum have completed 
their formation, on the fifth day, a narrow strap-like process- 
of the ectoderm of the stomodseum grows out over the endo- 
dermic part of each filament (Text-fig. 50, Ect. M. F., and 
PI. 4, fig. 33, E. S. 0.; cf. Wilson's fig. 21 ( 15 )). In 
this ectodermic band, which is only 6 ju (or one section) broad 
at first, nematocysts, mucous and granular gland cells 
develop. The band, lying over the endodermic part of the 
filament, soon widens and forms an ectodermic layer covering 
the outer surface of the latter (Text-fig. 51). [In this figure 
the two layers in the filament are marked Ect. M. F. and End.. 
M.F., and are shown in transverse section, while Text-fig. 50 
represents a much earlier stage in longitudinal section (about 
stage of PI. 4, fig. 33), the dotted line marking the edge of 
the mesentery before the endodermic thickening began 
( Mes . E.). The letter W. marks the lower limit of the stomo- 
dseal wall ($.), before the ectodermic downgrowth developed.] 
By the sixth day these filaments extend half way down the 
free edge of the mesentery (PI. 4, fig. 25, S. 0.). During 
subsequent growth the filament becomes somewhat twisted 
laterally and gathered up into puckers on the edge of the 
mesentery as a result of growing faster than the latter.. 
Consequently, sometimes the ectoderm and sometimes the 
endoderm cells lie uppermost, and this can be realised easily 
by imagining the filament shown in transverse section in 
Text-fig. 51 twisted from side to side. Longitudinal sections 
through the convoluted filaments are therefore somewhat diffi- 
cult to understand at first sight, as groups of ectoderm and 
endoderm cells lie side by side. 1 
While the ectodermic portion is secretive, the endodermia 
1 A model in red and white clay of the endodermic and ectodermic- 
parts of the filament was similarly twisted from side to side and then 
sectioned to check this result. 
