POSITION OF OPTIC ANLAGE IN AMBLYSTOMA 



285 



capacity possessed by the ectoderm at the anterior tip of the 

 head. 



Lens-forming power seems to diminish from the anterior tip 

 of the head backward until trunk ectoderm no longer possesses 

 the capacity to form a lens, as Spemann found in transplantation 

 experiments. For this reason independent lenses arise, as a rule, 

 far anterior to and often in front of the more or less lateral eyes 

 (my figs. 3, 4, 7, 9, 10, 11, 12, 21, etc. '10 b). In fewer cases 

 independent lenses are found in more posterior lateral regions 

 approximately in the normal lateral eye position (figs. 1 and 2 



Fig. 9 Two diagrams indicating the primary lens-forming power of various 

 portions of the head ectoderm. The lens-forming tendency is considered to be 

 greatest at the anterior end and gradually decreases towards the trunk ectoderm 

 until the ability to form a lens is lost where the trunk begins. The rows of circles 

 indicate the magnitudes of lens-forming tendency in different regions and do 

 not signify the size of the lenses. Posterior lenses may be as large as anterior 

 ones, yet they occur less frequently as independent structures. Free lenses 

 usually occur near the anterior tip. 



and 3, plates I and II '10 b). Figure 9 may serve to illustrate 

 diagrammatically the extent and gradation of the lens-forming 

 power possessed by the head ectoderm. This rather diffuse locali- 

 zation of lens-forming cells in the general head ectoderm as 

 demonstrated by numerous experiments seems sufficient to ac- 

 count for all phenomena of lens formation in cyclopia, as well as 

 the supernumerary lenses which the writer has reported. 



The median position in cyclopia of normally bilateral organs 

 involves one other part. The nose or nasal pits in the cyclopean 



THE AMERICAX JOURNAL OF ANATOMY, VOL. 15, NO. 3 



