The Nervous System of Amblystoma. 445 



anterior depression a neural groove since it forms before any 

 well defined anlage of the nervous system appears and when 

 its later history, as will be shown, shows no particular relation 

 to the nervous system. The term germinal depression does not, 

 by begging the question at the very outset, confuse the problem 

 of the morphology of the various grooves, but leaves the field 

 clear in which to follow the history of these important struc- 

 tures. 



Stage 3 (Fig. 1, C. Plate I, fig. 2)). This stage shows the 

 three early grooves present at one time. At the anterior end 

 lies the anterior germinal depression, in the middle the posterior 

 germinal depression, and at the posterior end of the series the 

 blastogroove. These three grooves are distinguished from one 

 another not only by order of appearance and position but also 

 by nature of the grooves themselves as will be shown later by 

 means of transverse sections. 



The anterior germinal depression (agd, fig. 1, C) shows some 

 changes when compared with the preceding stages. It is narrower, 

 deeper and longer and is more closely united with the posterior 

 depression. In fact in this stage it is more difficult than at any 

 other time to distinguish between the two depressions. Yet 

 this is possible in every embryo, the majority showing the dis- 

 tinguishing features much better than the one illustrated in 

 Plate I, fig. 2. By reference to fig. 1, B and E, typical repre- 

 sentatives of their respective stages, it is seen that although it 

 may be difficult to locate the exact point of union of the two 

 grooves, yet the grooves are readily distinguishable from each 

 other by a difference in width and depth. 



The posterior germinal depression shows further signs of 

 degeneration by widening out and disappearing at the posterior 

 end. Scattered shallow pits again appear in an irregular manner 

 as in the first stage 



The blastogroove is formed by the concresence of the lateral 

 lips of the blastopore. This groove together with the neuren- 

 teric canal can be described better in connection with one of the 

 older stages. It may be noticed here that the neurenteric canal 

 (nee) forms the anterior limit of the blastogroove and that the 



