394 EYCLESHYMER. [Vol. X. 



made up of narrow elongated cells which are deeply pigmented 

 in their outer ends. 



With the elongation of the embryo the neural folds become 

 more prominent (PI. XXII, Fig. 2). They are widely sepa- 

 rated anteriorly, while posteriorly they are beginning to close. 

 Certain markings which might be interpreted as neuromeres 

 are often observed in the neural folds, yet their arrangement 

 is decidedly irregular, and one is led to believe that they in- 

 dicate nothing more than artifacts caused by the killing re- 

 agents. 



The folds close more rapidly at the anterior end, so that in 

 the succeeding stage (Fig. 3) they are about the same distance 

 apart, throughout their entire length; in the cephalic region of 

 the embryo, on either side there appear slight swellings. The 

 anterior of these is the forerunner of the primary fore-brain, 

 while the second represents the common foundation of the 

 second and third cerebral vesicles. The folds soon close 

 cephalad along their entire length. Occasionally I have seen 

 the folds meet first at a middle point, as described by Clarke ('80). 



In Fig. 4 the folds have closed, the outline of the embryo is 

 well defined, the cerebral vesicles are distinct, the myomeres 

 are differentiated throughout the greater portion of the em- 

 bryo. The cranial nerves are forming; in short, the embryo 

 is taking on those peculiarities which enable us to speak of it 

 as a larva. I wish now to return to the consideration of cer- 

 tain features but faintly foreshadowed in the development of 

 Amblystoma, namely: 



II. The Optic Vesicles. 



Many attempts to explain the inverted position of the retina 

 have led to the hypothesis that the vertebrate eye must have 

 been located, originally, within the brain, Lankester ('80), 

 Balfour ('85) ; or, as Beard ('88) stated : " Most of us now 

 accept the view of Balfour, Carri^re, and others, that the eyes 

 were once structures opening dorsally on the surface of the 

 unclosed neural plate." 



While this was undoubtedly quite generally accepted, there 

 were no observations to this effect until I showed in a prelimi- 



