462 



Leland Griggs. 



depression (id) have been omitted. The retinal spots may as 

 Locy has claimed be a part of a longitudinal series of sensory 

 patches but Amblystoma shows only the first pair unless the 

 lateral infoldings have some such significance. In any case 

 both the retinal spots and the lateral infoldings lie in the peri- 

 pheral groove. The blastopore after it has been transformed 

 into the blastogroove also takes its place among the longitudinal 

 zones. The infundibular depression in its later development 

 includes the whole of the first neuromere and so marks that 

 transverse division to a certain extent as standing apart from 

 the other neuromeres. 



The following table gives concisely the arrangement of the 

 structures mentioned above. 



A. Longitudinal Divisions 



1. posterior germinal depression. 1. 



2. anterior germinal depression. 



3. blastogroove, developing from the 



blastopore. 2. 



4. neural plate. 3. 



5. peripheral groove, with lateral 



infoldings and retinal spots. 



6. neural crests. 4. 



7. neural groove, which forms where 



the two germinal depressions 

 and the blastogroove wholly or 

 in part disappear. 



B. Transverse Divisions 



procephalic lobes, a tagma marked 

 off by the transverse cephalic 

 groove. 



fourth neuromere. 



first three neuromeres, the first 

 later folding down as the infun- 

 dibular depression. 



neuromeres of medulla and spinal 

 cord, a region where tagmata 

 cannot be observed in Ambly- 

 stoma. 



Stage 7 (Fig. 6, 7; plate I, figs. 12-15). It has already been 

 shown that in front of the procephalic lobes the peripheral groove 

 becomes dee )er and wider forming a very conspicuous depression, 

 the infundibular depression (id, fig. 6, D). This grows deeper 

 and wider in older embryos until finally the entire first neuromere 

 is folded down and lies on the floor of the depression (nl, fig. 6, 

 C) where it remains permanently at a level considerably lower 

 than that of the other neuromeres. 



The whole process takes place very quickly with the resuly 

 that a very few embryos show intermediate stages. Nearly 

 all are in the condition shown in plate I, figs. 12-15 where three 

 very regular neuromeres are visible in the procephalic lobes^ 



