The Nervous System of Amblystoma. 



455 





/I 





B 



Fig. 4. Surface views of entire embryos to show the development of neuromeres, 

 sixth stage, agd, anterior germinal depression; id, infundibular depression; 

 li, lateral infolding; nl, n2, n3, n4, the four neuromeres of the procephalic lobes; 

 nc, neural crest; ng, neural groove; pel, procephalic lobes; rs, retinal spot; teg, 

 transverse cephalic groove. 



Stage 6 (Fig. 4, 5; Plate I, figs. 8-11). The following descrip- 

 tion of the development of the neuromeres in the neural plate 

 should perhaps be prefaced by a few statements showing the 

 evidence upon which it is based. The drawings are all made 

 from specimens killed and hardened in chromic acid but the 

 number and arrangment of the neuromeres has been confirmed 

 by a study of living eggs, as well as by eggs killed in other fluids 

 and by longitudinal sections. Four is the largest number of neu- 

 romeres found in the procephalic lobes. Where there are ess 

 than four there is always apparently room for the development 

 of the full number 



The first neuromere to appear lies just in front of the trans- 

 verse cephalic groove (n4, fig 4, A). Its anterior boundary is 

 formed like the transverse cephalic groove from a pair of pits 

 which develop in the peripheral groove and which in a few eggs 

 are visible in earlier stages (fig. 2, A). In this stage the groove 

 is seen in various degrees of development. It is nearly com- 

 plete in fig. 4, A and fully formed in fig. 5, A. The three remain- 

 ing neuromeres appear in the same way but do not follow^ any 

 definite order in their appearance. Fig. 4, A and B, represent 



