OUTLINI']S OF THE DEVELOP^[ENT OF THE TUATARA. 63 



the primitive streak. It agrees closely in histological character 

 with the primitive-streak mesohlast of the chick as described 

 by Marshall (7), but differs in its much greater forward 

 extension, and in the much larger share which it takes in the 

 formation of the embryo. In this connection it is interesting 

 to note that, according to Foster and Balfour (9), Kolliker 

 holds that the mesoblast of the region of the embryo is derived 

 from a forward growth of the primitive streak. 



In Lacerta mural is Balfour states (8) that a layer of 

 mesoblast spreads out in all directions from the primitive 

 streak, and is stated by Kupffer and Benecke to be continuous 

 across the middle line in the region of the embryo. Tliis 

 latter statement Balfour regards as highly improbable, but it 

 certainly agrees with what I have observed in Sphenodon 

 (compare fig. 11). 



" The chorda-entoblast " of Hertwig and Mitsukuri and Ishi- 

 kawa in Trionyx (5), the '^ axial strip of invaginated hypo- 

 blast" of Weldoii in Lacerta (6), and the 'Miead-process" of 

 Haswell in the emu (10), all seem to correspond to sonie 

 extent to the layer under discussion. 



Posteriorly the primitive streak passes insensibly into a 

 transverse thickening formed of precisely similar cells, and re- 

 presenting the so-called " sickle,^^ as described by Mitsukuri 

 and Ishikawa (5) in Trionyx. From the sides of the primi- 

 tive streak, or one might say from the ends of the transverse 

 ''sickle," two lateral wings of similar cells (mesoblastic) grow 

 forwards and slightly outwards (at Stage E), outside the 

 embryo proper, and give rise to the vitelline vessels in the 

 vascular area (fig, 20, L. W.). 



In the region of the primitive streak the dorsal surface of the 

 embryo sinks to the general level of the blastoderm, and here 

 the blastopore is established at Stage C by invagination, some 

 little way behind the medullary plate (fig. 4, 5, P.). This 

 invagination, in the form of a pit lined by columnar cells, 

 gradually extends downwards, and at Stage F comes to open 

 by a single well-defined aperture into the space (the so-called 

 segmentation cavity) beneath ihe embryo, which will presently 



