646 EARLY DEVELOPMENT OF THE LACERTILIA. 



preserved I feel very confident in asserting the continuity of the 

 notochord with the hypoblast at this stage. 



At the hind end of the embryo is placed a thickened ridge of 

 tissue which continues the embryonic axis. In this ridge all the 

 layers coalesce, and I therefore take it to be equivalent to the 

 primitive streak of the avian blastoderm. It is somewhat triangu- 

 lar in shape, with the apex directed backward, the broad base 

 placed in front. 



At the junction between the primitive streak and the blasto- 

 derm is situated a passage, open at both extremities, leading 

 from the upper surface of the blastoderm obliquely forwards to 

 the lower. 



The dorsal and anterior wall of this passage is formed of a 

 distinct epithelial layer, continuous at its upper extremity with 

 the epiblast, and at its lower with the notochordal plate, so that 

 it forms a layer of cells connecting together the epiblast and 

 hypoblast. The hinder and lower wall of the passage is formed 

 by the cells of the primitive streak, which only assume a colum- 

 nar form near the dorsal opening of the passage (vide fig. 4). 

 This passage is clearly the blind sac of Kupffer and Benecke, 

 who, if I am not mistaken, have overlooked its lower opening. 

 As I hope to show in the sequel, it is also the equivalent of the 

 neurenteric passage, which connects the neural and alimentary 

 canals in the Ichthyopsida, and therefore represents the blasto- 

 pore of Amphioxus, Amphibians, &c. 



Series A, figs. 2, 3, 4, 5, illustrate the features of the passage 

 and its relation to the embryo. 



Fig. 2 passes through the ventral opening of the passage. 

 The notochordal plate (ch'.} is vaulted over the opening, and on 

 the left side is continuous with the mesoblast as well as the 

 hypoblast. Figs. 3 and 4 are taken through the middle part of 

 the passage (tie.), which is bounded above by a continuation of 

 the notochordal plate, and below by the tissue of the primitive 

 streak. The hypoblast (/#'.), in the middle line, is imperfectly 

 fused with the mesoblast of the primitive streak, which is now 

 continuous across the middle line. The medullary groove has 

 disappeared, but the medullary plate (m p.} is quite distinct 



In fig. 5 is seen the dorsal opening of the passage (ne,}. If 

 a section behind this had been figured, as is done for the next 



