No. 3-] THE VERTEBRATE HEAD. 529 



cial distinguishing features between those of the head and those 

 of the body region. The median plate included between the 

 neural ridges is smooth at this stage ; at a slightly later period, 

 however, while the groove is still widely open, the median 

 plate exhibits very faint transverse markings. 



The contrast between this condition in Amblystoma and 

 that in a closely related Qg% (Rana palustris) is very instruc- 

 tive. In the latter (Fig. 115) the cephalic plate is very obvi- 

 ously segmented, while the folds in the neural ridges are 

 extremely difficult to see. We thus have in these two closely 

 related eggs strikingly different conditions. In the one it is 

 the median plate material that is thrown into obvious folds, 

 while those in the neural ridges are well-nigh indiscernible; 

 and in the other the conditions are reversed. This is not, 

 however, to be taken as indicating profound differences; for a 

 little careful observation shows that the median divisions do 

 not correspond to those in the neural ridges, and therefore 

 the median folds in Rana palustris are not to be compared to 

 the primitive segmental folds in the neural ridges of Amblys- 

 toma. Careful observation shows, also, that there are seg- 

 mental divisions in the neural ridges of Rana palustris that 

 do correspond, in number and general characteristics, with 

 those in the neural ridges of Amblystoma. These latter seg- 

 mental divisions are extremely difficult to see in Rana palustris. 

 There are four or five median transverse divisions in the 

 cephalic plate of that form, while there are ten or eleven 

 segments in the neural ridges of the same region. 



These facts throw light upon the apparent discrepancy be- 

 tween the observations of Kupffer and Froriep and those I 

 published in a preliminary article. Both the former authors 

 observed segments in the median plate of amphibian embryos, 

 and none in the neural ridges, while I have figured segments 

 in the neural ridges of Amblystoma, and none in the median 

 plate. But my later observations show that the appearances 

 even in closely related eggs of the same age are not necessarily 

 identical. 



Froriep agrees with Kupffer as to the position of the seg- 

 ments in the early stages, but not as to their number. In 



