OUTLINES -OF THM DEVELOPMENT OF THE TUATAEA. 61 



Stage, so that already at Stage C, and probably before^ the yolk 

 is completely enclosed. Tliis rapid spreading of the blastoderm 

 appears also to be characteristic of lizards^ and chelonia,^ and 

 perhaps of reptiles generally as compared with birds. 



At Stage C, the earliest examined, the blastoderm is some- 

 what vaguely divided into area pellucida and area opaca, with 

 the cap-shaped embryo lying in the former (figs. 1, 2, 4). 



In the area opaca the blastoderm consists of two distinct 

 layers, an upper epiblast, consisting of a single well-defined 

 layer of flattened or columnar polygonal cells (fig. 4, ^J.),and 

 a lower multiple layer of irregular stellate cells with numerous 

 yolk particles entangled in their meshes (fig. 4, L. L. Y.), 

 This lower layer evidently corresponds to the lower layer of 

 the chick, though it seems to be thicker and less sharply 

 defined from the yolk. A similar layer in Chelonians is 

 spoken of by Mitsukuri and Ishikawa (5) as " the yolk " con- 

 taining nuclei ; but in Sphenodon it separates readily from 

 the underlying yolk, which is distinguished from it by its 

 want of coherence. 



In the area pellucida the epiblast cells gradually become 

 more columnar in character as they approach the embryo, but 

 outside the embryo itself they still remain as a single layer 

 (fig. 4). 



The lower layer in the area pellucida becomes divided into 

 two, which are separated from one another by a large cavity 

 extending right across beneath the embryo (fig. 4, aS*. C). This 

 cavity evidently corresponds to the similar cavity in the chick, 

 termed by Foster and Balfour (9) the segmentation cavity, 

 and by Marshall (7) the subgerminal cavity. According to 

 Weldon (6) it appears to be doubtful whether or not such a 

 cavity occurs in Lacerta. 



The floor of the segmentation cavity in Sphenodon is 

 formed by a thin membrane which I have termed the sub- 

 embryonal membrane, and which is represented by the dotted 

 line in fig. 4. In the earlier stages of development it is 



> Compare Balfour (8). 



2 Compare Mitsukuri and Ishikawa (5). 



