No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 397 



widely separated. There is but little change in the histologi- 

 cal character of the cells, except that their boundaries are less 

 distinct. Owing to division and loss of yolk, the cells of the 

 embryo have gradually become smaller and more compact. 

 The originally broad space enclosed between the neural folds 

 has undergone a constant reduction in size; a rapid closing of 

 the anterior portion has taken place, so that along the entire 

 length of the embryo the folds are approximated. 



A section at this stage is represented in Fig. 14. The canal 

 is elliptical owing to the slight evagination of the optic areas 

 which are the forerunners of the optic vesicles. Their bilater- 

 ality is indicated by the distribution of pigment only, and one 

 might justly consider them as derived from a common basis. 

 A marked migration of the pigment has taken place; instead 

 of being located at the ends of the cells as in earlier stages, it 

 is found between them and nearer the periphery. The nuclei 

 have likewise undergone a further migration toward the sur- 

 face, so that the cells of the superficial layer have completely 

 lost their identity. 



It is of interest to note that in Petromyzon at a stage cor- 

 responding closely to the above, Kupffer ('90) has observed 

 that an unpaired basis for the eyes is present as the following 

 quotations show : " In der Mittelebene zeigt sich gar kein 

 Merkzeichen welches auf Duplicitat der Anlage deutet, dieselbe 

 ist vielmehr zunachst eine unpaarige. . . . Die spater paarige 

 Erscheinung des Organs wird nur darin angedeutet, dass in 

 den lateralen Regionen der Erweiterung beiderseits Mitosen 

 auftreten, die am Boden fehlen." 



During the later stages of development the walls of the vesicles 

 become thinner so that just before they invaginate to form the 

 optic cups they consist of but a single layer of elongated cells 

 with their nuclei located in their peripheral ends. A continued 

 dispersion and migration of the pigment takes place, and at this 

 time is more abundant in the stalk than in the portion which 

 will later form the optic cup. The later development of the 

 vesicles in this form is essentially the same as in Necturus and 

 Amblystoma. Since these stages have been carefully studied by 

 Professor Mall ('93) a further description would be superfluous. 



