OUTfJN'KS OF THK HEVRLorMKNT OF THE TUATARA. 15 



meshwork of stellate cells ; it forms part of the lo\ver layer, 

 and is continuous round tlie margin of the segmentation cavity 

 with the similar hut much thicker layer which elsewhere lies 

 immediately heneath the epiblast. The roof of the segmenta- 

 tion cavity is formed by the area pellucida, which, however, is 

 not at all sharply distinguished from the more opaque area 

 which surrounds it. 



In the hinder part of the area pellucida the embryo appears 

 as a cap-shaped structure (fig. 1) projecting above the general 

 surface of the blastoderm, from which it is marked off by a 

 deep fold shaped like a horseshoe, very pronounced at the 

 broad anterior end, but gradually dying out to the general 

 level of the blastoderm behind. This is evidently the head- 

 fold (figs. 1, 3, 5,H.F.). 



Towards the close of this stage a distinct blastopore (figs. 1, 

 3, 4, B. P.), with a well-defined crescentic anterior margin, 

 appears at the posterior narrow end of the embryo. 



The embryo at this stage is very difficult to remove and 

 harden satisfactorily. Kleinenberg's picric acid does not seem 

 to be altogether suitable for the purpose. Owing, doubtless, 

 to contraction, em.bryos 6 and 8 became cracked on top, while 

 No. 7 appears to have shrunk inwards, so that it is now 

 concave above and convex below, instead of just the opposite. 

 Nos. 5 and 9, however, were satisfactorily preserved. Tbe 

 former was mounted whole in Canada balsam, and the latter 

 cut into longitudinal sections. 



A median longitudinal section passing through the blasto- 

 pore has tlie appearance shown in fig. 4. It will be seen that 

 in the area pellucida around the embryo the epiblastic cells 

 gradually lose their flattened character and become elongated 

 radially, prismatic, with very conspicuous, very darkly staining 

 nuclei. Beneath them the lower-layer cells, here free or nearly 

 free from yolk, form a fairly compact multiple layer. The 

 lower limit of this inner layer is perfectly definite, but no very 

 distinct hypoblast can yet be said to exist, though it will be 

 formed later on from the deepest of the cells. (The cells of 

 the sub-embryonal membrane, in the floor of the segmentation 



