CURTIS: LIFE HISTORY OF PLANARIA MACULATA. 535 
same season of the year from localities where sexual reproduction 
is in progress and fission is practically absent, formative cells may 
be found dividing, but only by considerable searching. What part 
these cells play in the regeneration of the new parts after normal 
fission, will appear from the following description of the histolo¬ 
gical changes found in the head and tail pieces during the four or five 
days after their separation. 
The figures (pi. 13, figs. 35, 36 ; pi. 14, figs. 37, 38 ; pi. 15, fig. 42) 
upon which this description is based, are all from sagittal sections 
of tail pieces which are better adapted to illustrate the facts than 
sections in any other plane. The changes in the posterior-part of 
the head'ends are exactly the same in character minus, of course, 
the formation of the organs peculiar to the head; and any one of 
the figures might do as well to represent what occurs in the poste¬ 
rior end of a head piece. In all the figures of plates 13, 14, 15, 
except figures 39 and 42, the anterior end of the animal lies to the 
left. In figures 39 and 42, the anterior end of the animal lies to the 
right. Figure 35, of plate 13, represents a sagittal section of the 
anterior portion of a tail piece such as is shown in plate 9, figure 2. 
Figure 36 (plate 13) is a similar section of a tail piece about thirty 
hours after fission (pi. 9, fig. 3). Figure 38 (plate 14) is part of a 
sagittal section from about halfway between the anterior and poste¬ 
rior ends of a tail piece which has just separated from its head, 
but it might equally well represent any vertical, longitudinal section 
toward the middle of any animal from a locality where active fission 
is occurring. Figure 42 (plate 15) represents a sagittal section 
through just the tip anterior end of a tail piece on the third day after 
fission (pi. 9, fig. 4). Figure 39 (plate 15) is a section similar to 
figure 38 (plate 14) but is in the region of the developing pharynx 
(pi. 9, fig. 5). In this figure, the anterior end is to the right. Fig¬ 
ure 40 (plate 15) is a section in which the anterior end lies to the 
left and shows the pharynx fairly well formed (pi. 9, fig. 17). Fig¬ 
ure 37 (plate 14) is again a section similar to figures 35 and 36 and 
represents what is found at the anterior tip on the fourth day 
(pi. 9, fig. 5). 
The conditions found in the naked tip of the tail piece (pi. 9, fig. 
2) a few hours after its separation from the head piece, are repre¬ 
sented by the sagittal section shown in figure 35 (plate 13). Dor- 
sally, the formative cells (s) are seen to lie in the parenchyma and 
