458 F. G. HEATHCOTE. 
which the embryo is finally hatched. The somites are at first 
solid, but a cavity appears in them at a later period. 
The ectoderm of the ventral plate now alters its character, the 
cells becoming more pointed and much more closely packed 
together. 
From the Formation of the Stomodeum and Proc- 
todeum to the Hatching of the Embryo. 
Early on the ninth day the stomodzum is formed as an in- 
vagination of the ectoderm near one end of the ventral surface. 
Shortly after the first formation of the stomodzeum the proc- 
todzeum appears as a shallow, somewhat wide invagination near 
the end of the ventral surface. 
The body segments, already established by the segmentation 
of the mesoderm, now become more apparent, each being 
marked by a deep transverse furrow in the ectoderm (figs. 24, 
25,28). Fig. 12 is a section taken longitudinally through the 
embryo, and shews the stomodeum, the proctodeum, the eight 
mesodermal segments, and a single ectodermal furrow close 
behind the stomodeeum. Fig. 24 shows this first furrow under 
a higher power. (Zeiss c.) 
The endoderm cells are still scattered within the yolk, but 
they are gradually becoming collected in the median line just 
below the mesoderm. The stomodzeum and _ proctodeum 
become more deeply invaginated, extending a considerable 
distance into the yolk and at the same time the endoderm cells 
begin to form the mesenteron, arranging themselves round a 
central lumen. Fig. 27 shows the formation of the proc- 
todeum and the hypoblast cells beginning to form the 
mesenteron. 
At the end of the ninth day, then, the embryo is of a long 
oval shape, with a deeply invaginated stomodeum at the 
anterior end and a proctodeum not quite so deep at the other; 
the mesoderm is divided into eight segments ; a deep furrow in 
the ectoderm marks off the first segment which will eventually 
become the head, and the mesenteron is almost formed. 
