CURTIS: LIFE HISTORY OF PLANARIA MACULATA. 
539 
all become oriented alike and so closely packed that in sections 5 /x 
in thickness they are sometimes two or three cells deep. This mass 
of cells {ph') covers the extent of the new pharynx and there 
can be no doubt as to what cells are responsible for this organ. 
Between the anterior end of this mass and the gut rami which have 
now united (pi. 9, fig. 5), are fewer cells also markedly spindle- 
shaped and drawn out in the longitudinal axis of the animal (pi. 15, 
fig. 39, Apparently the fibers of the parenchyma about these 
are also drawn out, for they appear different from those surrounding 
them and this differentiation marks out the future lumen connecting 
gut with pharynx. Where it meets the gut (< 7 ), there is such an 
intimate association between the gut and the formative cells (s) that 
one is forced to the conclusion that these are being added on to the 
gut. This apparent fusion of the formative cells with parts of the 
gut will be taken up in detail later. The mass of closely packed 
cells extends about twice as far back as is shown in figure 39. The 
cavities of the pharynx lumen and its sheath have not yet appeared. 
The cavity of the sheath appears a little before the cavity of the 
lumen, being first evident as a split around the periphery of the 
posterior two thirds of the mass of formative cells (pi. 15, fig. 39), 
then the lumen begins to appear by a separating of the individual 
cells in the center. 
Figure 40 (plate 15), in which the anterior end of the animal lies 
to the left, represents a stage in which the lumen {plil') of the phar¬ 
ynx is forming and is not connected with the gut (^) nor does it 
open on the free end of the pharynx. The pharyngeal sheath is 
formed, but the mouth has not broken through. The mass of forma¬ 
tive cells of the third day (pi. 15, fig. 39) is here represented by 
many rounded cells in the rudiment of the pharynx. Many of these 
show plainly their cell outline and a nucleus with the characteristic 
nucleolus of the formative cell. In the center they have separated 
for the beginning of the lumen {phi') the epithelial lining of which 
is just beginning to form, and over this area the parenchymatous 
matrix has disappeared. The epithelium of the pharyngeal pocket 
is already formed, but some of its nuclei still show the characteristic 
nucleolus of the formative cell. Just anterior to the j^harynx in 
this figure are to be seen what are unquestionably formative cells 
which are within the gut cytoplasm {g) and in the neighboring 
sections this approaches almost to the diminutive lumen {pliV). 
