538 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
This section cuts through a considerable portion of one of the still 
unfiised gut rami (pi. 9, fig. 3). 
The third day (pi. 15, fig. 42) shows an increase in the number 
of small nuclei evidently arising by further division of the large, 
formative cells so that the latter are found in considerable numbers 
only some distance back. The ectoderm at the exposed tip (ec') is 
now definitely formed and save for the absence of rhabdites, the 
boundarv between the old and the new ectoderm could not be dis- 
tinguished. The killing fluids often produced clefts between the 
new ectoderm cells, as shown in the figure, a phenomenon which I 
have frequently observed in the delicate ectoderm of embryos killed 
when about ready to leave the egg capsule. 
On the fourth day (pi. 9, figs. 5, IT) a similar section through the 
head (pi. 14, flg. 37) shows approximately the mature organization 
of the parenchyma in this region, there being many small nuclei and 
the larger cells being entirely absent until a point opposite the gut 
is reached. This is a section of the anterior end of a tail piece con¬ 
siderably smaller than the others and as it is drawn to, the same 
scale, it might at first glance indicate more elongation and flatten¬ 
ing of the new tissue than occurs. There is, however, on the third 
and fourth days, a flattening and elongation which does change the 
shape of the regenerating end as the outlines of the figures show. 
The commissure (?ie) connecting the two halves of the new brain 
is now seen for the first time. 
The formative cells at the exposed tip seem to form the new ecto¬ 
derm as above described, and the large number of small nuclei of 
the adult head parenchyma to all appearances come from the divi¬ 
sion of the nuclei of formative cells, the cytoplasm of which 
becomes continuous with that of the parenchyma syncytium (pi. 15, 
fig. 41, S?2) . 
For as much as twenty-four hours after the fission there is no dis¬ 
coverable change in the parenchyma at the point where the new phar¬ 
ynx is to form. During the second day, there appears in this region 
an irregular massing together of large, formative cells. Dorsal and 
ventral to this mass of cells, the mitotic figures are abundant, but 
are not so frequent in the mass itself. The appearance of so many 
large cells all at once suggests again that some have migrated from 
the dorsal mass of formation cells. * By the third day (pi. 15, fig. 39) 
these irregularly directed oval and spindle-shaped cells have almost 
