1895.] Embryology. 181 
and separated from one another on the surface by deutoplasm. The 
nucleusin the germinal dise divides, and its descendants scattering in 
the protoplasm form a syncitium adherent to the surface of the food 
yolk. As the disc spreads, the central protoplasm divides up into cells 
around its nuclei, while the periphery remains a syncitium. This syn- 
citium is soon broken up into cells, and as the blastoderm thus formed 
extends, it annexes more formative material as it reaches the islands 
of protoplasm. The periphery of the blastoderm is thus continually 
added to, and the protoplasm forming the growing edge is constantly 
supplied with nuclei from the centre of the disc, around which in their 
turn new cells are formed. Besides the protoplasm added from the 
islands, formative material is also supplied from the centre of the 
ovum among the deutoplasm. In this way the blastoderm gradually 
extends over the egg from the anterior pole ventrally and laterally, 
until it finally closes in at the posterior pole near the dorsal surface. 
(This type of segmentation seems to fall between Korschelt and 
Heider’s III b. and IV.) Soon after the first cells of the blastoderm 
have become well defined at the anterior pole, they begin to split off 
an under layer, the mesentoderm (Roule’s protendoderm). This layer 
is steadily added to as the blastoderm spreads, by a repetition of this pro- 
cess of splitting tangentially, by incorporating new cells thus formed at 
the anterior edge of the blastoderm, and by a division of its own cells. 
The anterior pole of the egg is the growing point of the mesentoderm. 
The cells lie for the most part along either side of the ventral mid-line, 
in two ridges projecting into the yolk, in the anterior region of the 
embryo. The cells being amoeboid, however, wander dorsally and 
posteriorly, where they lie as elsewhere in the outer portions of the 
deutoplasm. By this time the cells of the ectoderm have flattened, 
except in two regions, into a single layer. At one place where the 
brain is to appear, a rapid division of nuclei and fusion of cells takes 
place, until a syncitium is formed projecting into the yolk. A similar 
syncitial mass is formed on the ventral mid-line, to be the ventral cord. 
These two areas are connected by ectoderm, which will later be the 
esophageal ring. Nineteen pairs of appendages gradually appear 
from before backward. At first they are formed of an ectodermic 
sheath with a core of nutritive yolk and scattered mesentoderm cells; 
but soon the nutritive yolk is absorbed, leaving merely the mesoderm 
cells within. While the nervous system is becoming more distinct, 
and the appendages are growing and increasing in numbers, a procto- 
deal invagination, which appeared atthe point where the blastodermic 
disc closed in, and a stomodeal invagination, which appeared at the 
