42.2 F. M. BALFOUR. 



axis-cord. A fourth mass applied itself to the walls of the sac 

 formed by invagination. 



With reference to the very first developmental phenomena my 

 observations are confined to two stages during the segmentation.^ 

 In the earliest of these the segmentation was about half completed, 

 in the later one it was nearly over. My observations on these 

 stages bear out generally the statements of Kupffer and Benecke. 

 In the second of them the blastoderm was already imperfectly 

 divided into two layers — a superficial epiblastic layer formed of a 

 single row of cells, and a layer below this several rows deep. 

 Below this layer fresh segments were obviously being added to 

 the blastoderm from the subjacent yolk. 



Between the second of these blastoderms and my next stage 

 there is a considerable gap. The medullary plate is just 

 established, and is marked by a shallow groove which becomes 

 deeper in front. A section through the embryo is represented 

 in PI. XIX, Series a, fig. 1. In this figure there may be 

 seen the thickened medullary plate with a shallow medullary 

 groove, below which are two independent plates of mesoblast 

 {me. p.), one on each side of the middle line, very imperfectly 

 divided into somatopleuric and splanchnopleuric layers. Below 

 the mesoblast is a continuous layer of hypoblast (%.), which 

 develops a rod-like thickening along the axial line (c/i.). This 

 rod becomes in the next stage the notochord. Although this 

 embryo is not well 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, a7id I therefore take it to be equivalent to the 

 primitive streak of the avian blastoderm. It is somewhat 

 triangular 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 formsalayer of cells connecting together the epiblast and hypo- 

 blast. The hinder and lower wall of the passage is formed by the 

 cells of the primitive streak, which only assume a columnar form 

 near the dorsal opening of the passage {vide fig. 4). This passage 

 is clearly the blind sac of Kupffer and Benecke, whoj if I am not 



' Eor these two specimens, which were hardened in picric acid, I am 

 indebted to Br. Kleueinberg. 



