50 ARTHUR DENDY. 



spicuous externally, forming a broad rim around the lens, 

 interrupted by the choroid fissure. 



The parietal eye and its ''stalk" are both distinct, the eye 

 lying upon the roof of the brain, in front of and to the left of the 

 forward-pointing '' stalk,'' the cavity of which still communi- 

 cates widely with that of the brain. The eye is almost, if not 

 quite, separated from the " stalk," though still in close contact 

 with it, and its front wall has begun to thicken to form the lens. 



The paraphysis appears as a simple backward-pointing diver- 

 ticulum of the roof of the fore-brain, some distance in front 

 of the parietal eye, the two being widely separated by the greater 

 part of the thin roof of the thalamencephalon. 



Another noteworthy feature of this stage is the condition of 

 the anterior part of the notochord, which is twisted into a 

 spiral beneath the hind brain, while its narrowed tip bends 

 sharply down and ends just in front of the pituitary invagina- 

 tion, as shown in fig. 95.^ 



About this stage also a feature which becomes very con- 

 spicuous later on in the yolk-sac outside the embryo begins to 

 make its appearance. This consists in the formation of radial 

 corrugations of the yolk attached to the under surface of the 

 yolk-sac just outside the clear area (compare Stage O, fig. 96, 

 R. C), due to the development of the absorbent vessels in 

 connection with the vitelline circulation. Around these 

 vessels the yolk particles are aggregated as described in the 

 Introduction. 



Stage O (figs. 96—100). 

 To this stage I refer four embryos, viz. : 



89 ■) Collected on Stephens Island about the end of December, 1897, and 



90 i removed from the eggs on January 6th, 1898. 



92. Collected about the same time, and removed from the egg on January 

 10th. 

 103. Collected about the same time, and removed from the egg on January 25th. 



The following features may be taken as more or less charac- 

 teristic of this stage : 



' Cf. Parker, W. E., on the notochord in the embryo of the green turtle 

 ' Challenger Reports,' Zoology, vol. i. 



