196 WILSON. [Vou. II. 
derm segments are very hard to make out. The delamination 
takes place irregularly over the general surface of the blasto- 
sphere, and is of a peculiarly complex character in certain spots. 
In the cell groups # and m!, Fig. 2, for instance, the lines of 
the transverse divisions of the several cells are continuous, and 
seem to indicate that the whole group must have divided trans- 
versely while it was yet a single cell: before it had been split 
up by the longitudinal divisions. But the latter show that the 
transverse divisions did not precede them. Such a cell as x, 
Fig. 3, throws light on the matter. This cell has begun to 
divide longitudinally, but even in the peripheral part of the cell, 
though the longitudinal constrictions are evident enough, the 
segments are far more intimately connected than are, say 7, and 
the adjacent cells. In another cell, z', the longitudinal division 
is apparently perfect ; but on comparing the close apposition of 
the new segments with the marked space between z' and the 
adjacent cells, I am inclined to believe that the halves of z' are 
still connected by bridges of tissue, much as the blastomeres in 
many segmenting eggs remain connected after they are clearly 
marked off. In the group m!', then, Fig. 2, it is probable that 
the longitudinal constrictions were the first to appear in the 
mother cell, but that before they became complete divisions a 
general transverse constriction took place. 
Delamination appears to be the exclusive means by which the 
endoderm is formed. I looked in vain for cells migrating from 
the surface. Whenever there appeared to be such, examination 
of the next sections showed that the cells in question had been 
cut obliquely, the peripheral end with the nucleus having been 
severed from the rest of the cell body. The cell a, Fig. 2, for 
instance, is merely the central portion of a constricted cell like 
6 in the same figure. The delamination and accompanying 
longitudinal division often give rise to spheroidal cells, such as 
x, Fig. 3. These cells are always at the surface of the blasto- 
spheres. Whether their fate is different from that of the 
general ectoderm cells, I do not know. 
The cavity of the blastosphere having been filled up by the 
endoderm segments, a solid planula, Fig, 4, is formed. On 
studying this figure, a median longitudinal section, we see that 
the longitudinal division of the superficial cells, which was 
active in the blastosphere, has given rise to a layer of columnar 
