286 
LILIAN SHELDON. 
embedded in a loose reticulum of protoplasm is present in 
about the posterior half of the ovum ; this condition is shown 
in transverse section in fig. 3. The nuclei at the sides of the 
area also proliferate, as is shown in fig. 3 a , which represents 
a transverse section through this region from the same ovum 
as fig. 3, but is drawn under a higher power. 
The proliferating mass of nuclei increases in size and 
occupies a larger portion of the surface of the ovum, and 
both nuclei and protoplasm are closer and more compact ; a 
transverse section through the posterior half of such an ovum 
is shown in fig. 8. At the same time the protoplasm at the 
sides of the uncovered area become slightly inflected ; this is 
shown in fig. 9, which is a transverse section through this 
region from the same ovum as fig. 8, but drawn under a 
higher power. The uncovered area thus forms a passage 
lined by the cells of the blastoderm, which have become in- 
flected, and leading into the yolk ; it may be spoken of as the 
blastopore, as is the case in P. capensis it is traversed by 
strands of protoplasm. 
These two structures, i. e. the blastopore and the area of 
proliferating cells lying posterior to it, soon acquire very close 
resemblance to the blastopore and primitive streak of P. 
capensis, with which they are probably homologous. The 
blastopore increases in length and the protoplasm at its sides 
shows a true invaginate character, and a groove is present 
running from the posterior lip of the blastopore down the 
centre of the primitive streak. Three transverse sections 
through an egg of this stage are shown in figs. 5, 6, and 7. 
Fig. 5 passes through the blastopore at about the middle point 
of its length, where it is very clearly open ; at this time it is 
about an eighth of the total length of the ovum. Fig. 6 
passes through the region immediately behind the blastopore 
through the primitive streak and groove. Immediately beneath 
the primitive groove there is a small cavity bounded by the 
protoplasm of the primitive streak, the nuclei round it being 
arranged in a roughly columnar manner, and filled with very 
small yolk-spheres, among which are one or two nuclei ; this 
