94 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
from the rest of the yolk and lies on the periphery, where 
it continues to divide, and so forms a cap of blastoderm 
cells, no cleavage cells remaining in the food yolk. The 
starting point corresponds to the point where invagination 
takes place later, that being at the posterior end of the 
ventral side of the embryo. 
After the formation of the blastoderm a thickening 
is formed, corresponding to the future ventral side. This 
thickening is the germ disc. Shortly afterwards, three 
divisions of the germ dise make their appearance. Two 
anterior paired portions (PI. IV., fig. 6, a.m.) represent the 
formative region of the mesoderm; a median thickening 
(end.) situated posterior to, and between these, represents 
the fundament of the endoderm. The germ band is next 
formed by a probable forward growth of the mesoblast 
rudiments below the ectoderm, the ectoderm increasing in 
thickness. Three pairs of buds arise; these are the 
rudiments of the limbs, and this stage (Pl. IV., fig. 7) 
corresponds to the Nauplius stage. Behind these rudi- 
ments, and in front of the anal aperture, is a mesoblastic 
area, termed the formative area (f.z.), from which the 
remaining segments of the body will develop. The 
arrangement of the mesoderm cells in the formative area 
is extremely regular. 
At the beginning of the formation of the mid-gut a 
number of cells (vitellophags) leave the endoderm and 
wander inwards; they do not take any part in the forma- 
tion of the mid-gut, but assist in the disintegration of the 
yolk. The mid-gut is formed from two layers of cells 
which arise from the endoderm rudiment; these lie below 
the germ band, and gradually grow round the yolk, each 
being concave on its inner surface. By means of a ventral 
median piece they unite in the anterior region, and finally 
enclose the yolk by growing round to the dorsal side, so 
