The Nervous System of Amblystoma. 



465 



5 



Fig. S. Entire embryos showing the first steps in the closing of the neural 

 canal, and the relation of the neuromeres to the developing cerebral vesicles, 

 stage eight, c, cerebellar crest; e, eye;fb, fore-brain; id, infundibular depression; 

 nl, n2, n3, n4, neuromeres of the procephalic lobe; ol, optic lobe; rs, retinal spot. 



are usually indistinct and in at least half the eggs of this stage 

 they cannot be seen at all. Fig. 6, B, illustrates the best speci- 

 men of the entire collection for this purpose, but other eggs 

 vary both in number of the neuromeres and the appearance of 

 the neuromeres. In examining the four embryos shown in fig. 

 6, it will be noticed that a few narrow neuromeres are present. 

 While such narrow neuromeres appear only rarely, when they 

 do appear they always alternate with the usual wide neuromeres 

 and there are no neuromeres intermediate in size between the 

 two forms. 



It might be expected that these postcephalic neuromeres would 

 be found in groups or tagmata like the procephalic neuromeres. 

 Thus we can speak theoretically of a metencephalic tagma, the 

 anlage of the medulla, and of a spinal cord tagma, the anlage 

 of the spinal cord, but in Amblystoma the postcephalic neuro- 

 meres are too rudimentary and transitory to discover any such 

 relations. 



As the neural crest closes over the plate all signs of segmenta- 

 tion behind the procephalic lobes disappear. No landmarks 

 are left that can be followed into the adult brain. 



Stage 8 (Figs. 8, 9, 10, 11). In this stage the closing of the 

 neural tube is completed. The crests meet first in front of the 



