198 ETCHAED ASSHETON. 



a lumen in a section through its posterior half, as shown at 

 P. GR, in fig. 25. 



The origin of this groove — the primitive groove — I shall 

 discuss in a later paragraph. 



The presence of this groove necessitates that the thickened 

 epiblast of the neural plate anteriorly, and the thickened epi- 

 blast of the floor of the groove posteriorly, should be at dif- 

 ferent levels. In this it is possible that any intrinsic growth 

 of the neural plate may tend to produce an overlapping, and 

 so give rise to what has been termed a notochordal canal. 



The secondary area of proliferation having now attained its 

 maximum length and most complicated form, it henceforth 

 tends to return to its original shape, and becomes less and less 

 like a typical primitive streak. 



In fig. 7 no very great change has taken place as regards the 

 area in question, with the exception that the primitive groove 

 is not nearly so deep or narrow. It resembles more closely in 

 transverse section the stage of fig. 5. Also the layer of meso- 

 blast cells on either side of the primitive groove are thicker 

 now than in fig. 6. 



Fig. 8 shows the area still at its greatest length, and figures 

 of sections taken through the anterior end and the middle 

 are given on Pl.-i^^ figs. 30 and 31, and should be compared 

 •with figures of the corresponding sections from the earlier 

 stages. 



The most marked difference is the entire absence 

 of the groove. Compare fig. 31 with fig. 26. 



The apparent canal into Hensen's node has gone, and the 

 outline of the embryonal area adjoining this region has 

 changed shape. In fig. 4 the outline is arched away from the 

 primitive streak. Id fig. 5 it is less arched, in figs. 6 and 7 it 

 is nearly straight, in fig. 8 it is becoming again curved. From 

 this moment the length of the secondary area of proliferation 

 becomes less. 



In fig. 10, in which five protovertebrae are visible, this area 

 has diminished to about one third of its former length. 



Figs. 32, 33, and 34 are three sections through such a speci- 



