410 ARTHUR ROBINSON. 
gular form as on the left side of fig. 18 H. Their peripheral 
extremities remain continuous with the hypoblast, and the 
cavities which they contain become larger. 
Still more posteriorly (fig. 18 F, Pl. XX VII) the inner por- 
tions of the paraxial mesoblastic plates become divided up into 
a number of stellate cells, and their outer parts assume a more 
epithelial arrangement. One hundred and thirty micromilli- 
metres behind the anterior extremity of the hypoblastic ridge 
the lateral portions of the paraxial mesoblastic plates are 
separated from the hypoblast, but they have become con- 
tinuous with the somatic and splanchnic mesoblast of the 
extra-embryonic area (fig. 18 F). 
The changes in the form of the hypoblastic ridge are also 
most readily observable in transverse sections. Anteriorly it 
is widely expanded, thickened at its lateral margins, and 
slightly curved (fig. 18D); more posteriorly, apparently by 
folding, it is converted into a wedge-shaped ridge (fig. 18 E) ; 
then it becomes reduced to a single row of rounded nuclei 
embedded in a small amount of protoplasm, and this thin 
layer is excluded from the vitelline cavity by a single layer of 
flat cells (fig. 18 F) ; still more posteriorly, whilst it retains the 
same characters, it resumes its place in the dorsal wall of the 
vitelline cavity, and in the last 40 u of its extent it is a broad 
curved plate similar to that represented in fig. 20, Pl. XX VII. 
At the end of the ninth day a longitudinal section of the 
embryonic area may be divided into four parts. 
1. From the anterior end of the embryonic area to the an- 
terior extremity of the hypoblastic ridge (from A to B, 
fig. 18 A). 
2. From the anterior to the posterior end of the expanded 
anterior portion of the hypoblastic ridge (from B to C, 
fig. 18 A). . 
3.. From the posterior end of 2 to the anterior end of the 
primitive streak. 
4, From the anterior end of the primitive streak to the pos- 
terior end of the embryonic area. 
During the tenth day, simultaneously with the appearance 
