The Nervous System of Amblystoma. 441 



circular blastopore there may be some that show the outline of 

 the neural plate and others that show no trace of the plate. 

 This difficulty becomes much greater when the several distinct 

 parts of the neural plate are taken into consideration. 



The following division into stages is based upon the develop- 

 ment of the particular structure which is made the special' object 

 of study in the general period under consideration. Thus the 

 eggs before the appearance of the neural plate are divided into 

 three stages; these stages depending upon the relative develop- 

 ment of the germinal depressions, not upon the condition of the 

 blastopore. After the appearance of the neural plate, however, 

 the peripheral groove, the transverse cephalic groove, the neural 

 crest and the neuromeres are successively the landmarks for 

 division and it is necessary to ignore the germinal grooves, for 

 they show many degrees of degeneration in each stage. After the 

 closing of the neural canal these peculiar conditions are no longer 

 found and assortment into stages becomes comparatively simple. 



The method described above will be made clear by the follow- 

 ing summary of stages. 



Division into Stages 



Stage 1. The posterior germinal depression has appeared 

 in front of the blastopore. The blastoporic rim is bounded by 

 a faint narrow groove. 



Stage 2. The anterior germinal depression has appeared and 

 the posterior depression shows signs of degeneration from behind 

 forward. 



Stage 3. The two germinal depressions appear very closely 

 united. In a few eggs the blastogroove is now formed. 



Stage 4. The area of the neural plate has become marked 

 off by the peripheral groove. The condition of the germinal 

 grooves and of the blastogroove varies. 



Stage 5. The neural crests have appeared and there are indi- 

 cations of the transverse cephalic groove. 



Stage 6. The procephalic lobes become divided into neuro- 

 meres. 



JOURNAL OF MORPHOLOGY, VOL. 21, NO. 3. 



