534 p. E. SMITH 



date the primitive fore-brain and mid-brain vesicles, Neal and 

 Warren that they do not. The problem, difficult because of 

 minuteness of the foldings and the danger of artifacts, will have to 

 be worked out also in terms of other correlated structures. I have 

 made no attempt to do this because of the difficulty of the tech- 

 nique in this form and because of the obhteration of cavities which 

 in other forms would be prominent. In Desmognathus the 'neuro- 

 meres' of the fore- and mid-brain appear at about the same time 

 as the primitive brain vesicles. From the evidence I have seen, 

 however, the primitive fore-brain vesicle antedates slightly the 

 smaller segments (neuromeres) . In the mid-brain both appear at 

 about the same time, so far as I can determine. 



Summary 



No evidence is found in the cephalic portion of the medullary 

 plate, of divisions to which a segmental value should be assigned. 



There are irregularities and folds in this portion of the medul- 

 lary plate which are normal but not constant. 



The primary fore-brain vesicle becomes secondarily divided into 

 three segments; the cephalic one is included in the telencephalon, 

 the other two in the diencephalon. 



The mid-brain vesicle is composed of two segments. These 

 appear at the same time as the primary vesicle. 



GENERAL MORPHOLOGY 



Fish ('94) noted the preservation of embryological characters 

 in the adult brain of Desmognathus. Herrick ('10) states that, 

 judging from the figures of Fish, the fore-brain of Desmognathus is 

 intermediate between that of a 17 mm. and a 35 mm. Amblystoma 

 larva in regard to the less extensive development of the primordium 

 hippocampi and nucleus medianus septi and the more extensive 

 septum ependymale. The papers of Herrick and Johnston have 

 been especially valuable and my indebtedness to them will be 

 evident. 



