412 
CHIYOMATSU ISH1KAWA. 
depression, transverse to the long axis of the egg, and divides it . 
into an upper circular (figs. 45 and 46, a.) (containing the 
gastrula cavity) and a lower oval one (figs. 45 and 46, b .). 
The cells around the blastopore become much elongated, and 
appear white by reflected light. The lower oval depression 
disappears, while the blastopore and the circular depression 
get much smaller (fig. 47). The white area around the blasto- 
pore shifts upwards and takes a definite triangular shape (fig. 
49, ab). The depression becomes shallow and the blastopore 
closes (figs. 50 and 53, g. to.). 
Germinal Layers. — With the formation of the gastrula, 
we can already distinguish the origin of the germ-layers. The 
endodermis formed from the invaginated cells of the gastrula, 
while the rest of the blastoderm gives origin to the ectoderm. 
The cells of the bottom of the cavity as well as those near the 
blastopore gradually elongate, as was already said, and give off, 
by continual division, the cells of the mesoderm (fig. 52, ms). 
The formation of the mesodermic cells is more vigorous at 
the forward edge of the cavity than on the floor, and the con- 
sequence is that the opening of the cavity is gradually lessened, 
as has been clearly shown by Reichenbach (ff). At the time 
of the closure of the gastrula, there is seen a mass of proto- 
plasmic elements, aggregated just below the superficial ecto- 
derm (fig. 53, w. y.), which is very likely to be compared to 
the white yolk-elements of Reichenbach. After the closure of 
the blastopore the endoderm cells gradually travel into the 
yolk-segments, and their nuclei become indistinguishable from 
those of the yolk (fig. 54, h y ). Whether there exists a definite 
cell-outline to each of the endoderm cells after they have 
removed into the yolk, or whether the cell outliue is lost, I 
cannot tell with certainty. 
At a region somewhat in front of the late blastopore, a fresh 
invagination takes place, which gives rise to the permanent 
anus (fig. 62, pd.). The triangular white patch becomes more 
definite, and there is formed on each side of it, in front, a 
circular elevation (figs. 55 and 56, md.), which later becomes 
the mandible. The oval area in front of these becomes circu- 
