No. 3-] THE VERTEBRATE HEAD. 527 



Up to this point the figures described have all been magni- 

 fied uniformly 45 diameters ; but on account of the increase in 

 size of the embryos it will be better to carry forward the 

 history of these segments with figures drawn on a smaller 

 scale. Accordingly Figs. 50-60, inclusive, are magnified only 

 ten diameters. 



Fig. 50 represents an embryo of the same age as that shown 

 in Fig. 32. 



Fig. 5 1 shows the entire embryo, partly dissected, of which 

 Fig. 43 is a portion more highly magnified. Behind the figure 

 is seen the line of fusion of the lips of the blastopore. 



Fig. 52 is a sketch of the embryo of which Fig. 47 is the 

 enlarged view of a partial dissection. They all show well the 

 segmented condition of the walls of the hind-brain. 



Soon after the age represented in Fig. 55 is reached, the 

 neural segments fade away. Figs. 57 and 60 (PI. XXIX), 

 represent the head region of older embryos in which the seg- 

 ments are no longer visible. 



Taken together, the figures now described give a compara- 

 tively full view of the neural segments in different ages. They 

 show them from their first appearance to the time they fade 

 away. We learn from this examination that the neural seg- 

 ments are established before any embryonic organs appear, and 

 that in the early stages they extend not only throughout the 

 length of the embryo, but into the embryonic rim. In the 

 earliest stages the segments are alike, and there is no struc- 

 tural distinction to be made between those in the head and 

 those in the trunk, or even those in the embryonic rim. 



In sagittal sections the neuromeres are well exhibited. Fig. 

 72 shows a section of a specimen just after the closure of the 

 neural groove in which the five neuromeres belonging to the 

 fore- and mid-bsain are exhibited. The second neuromere 

 coincides in the median plane with the neuropore. This is 

 also to be seen in surface view in Fig. 32. Very soon the 

 anterior brain regions become so much modified and expanded 

 that the original segmental divisions are no longer visible. 



Figs. 66-71 (PI. XXIX), show six successive sections of a 

 specimen slightly younger than the one just described. In 



