170 ALICE JOHNSON. 



of the primitive groove at an earlier stage, and corresponds in 

 position more or less with the future mouth. 



As the medullary folds approach one another the primitive 

 groove becomes gradually obliterated in the narrowing and 

 folding up of the medullary plate, and the primitive streak 

 remains only in the hind region. At the front end of this 

 reduced primitive streak the sides of the medullary plate come 

 together to form a solid mass instead of the thick-walled canal 

 that exists in front. This fact is illustrated in figs. 6, 7, 8, 

 and 9, which are taken from a series of transverse sections 

 through the hind end of an embryo. The medullary canal in 

 passing back round the hind end gradually loses its lumen 

 (vide fig. 6, where the medullary canal is seen above and the 

 solid mass of epiblast cells below). Further forwards on the 

 ventral surface this solid mass becomes fused with the under- 

 lying hypoblast cells and the lateral plates of mesoblast. The 

 primitive streak, thus constituted, forms a slightly pronounced 

 ridge on the surface of the embryo (vide fig. 7). Nearer the 

 blastopore the ridge is flatter (vide fig. 8). In fig. 9 the blasto- 

 pore itself is seen with the continuity of the layers at its lips. 



Fig. 10 shows the primitive streak, as seen in transverse 

 section, of an embryo with a distinct tail, rudiments of the 

 visceral clefts, &c. In figs. 11, 12, and 13 the primitive streak 

 of difi'erent stages is shown in longitudinal section, but it can- 

 not then be distinguished so clearly. 



I have been unable to find at any stage the neurenteric 

 canal mentioned by Scott and Osborn. 



In the course of the development, the medullary canal is 

 gradually difl"erentiated backwards out of the primitive streak, 

 and the hind gut, from being curved as seen in fig. 13, becomes 

 straight. 



The arrangement of the layers in the primitive streak of the 

 Newt at the stage represented in figs. 6 — 9 resembles closely 

 that described by Professor Balfour in the tail of the embryo 

 Lepidosteus.^ 



1 F. M. Balfour and W. N. Parker, " On the Structure and Development 

 of Lepidosteus." 'Phil. Trans, of the Roy. Soc.,' part ii, 1882. 



