266 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-spheresj among which are one or two nuclei ; this 



