92 PATTEN [Vol. I. 



16. Diagram of compound eye, constructed according to Grenacher's views. 



17. An ommatidium from a compound eye, constructed according to 

 Grenacher's statements ; he does not recognize the corneal hypodermis, and 

 separates the eye into two layers at b.tn}\ the dotted line, jr, shows the posi- 

 tion of the crystalline cone in certain cases where it appears to be absent. 



17a, is a cross section of the retinulae, showing the seven (or four) rods 

 which they are supposed to secrete. 



18. Diagram showing five stages in the transformation of a pair of 

 epithelial cells; m^ represents a myo-epithel cell connected with the sensory 

 cell s'. In II. and III., 5' becomes a neuro-epithel cell, and /«' has become a 

 contractile cell. In IV., the neuro-epithel cell unites with a neighboring sense- 

 cell by fine fibrillae. In V., the outer end of the neuro-epithel cell has lost its 

 sense-hairs, and is united to a neighboring sense-cell by numerous fibrillse. Its 

 outer end has become a true nerve-fibre, while its nucleated inner part develops 

 new fibres uniting it with neighboring ganglionic cells. These five changes 

 result in the transformation of the original epithel cell 5', into the multipolar 

 ganglionic one 5*. 



The three cells 5', s* and /w* represent the three extremes in the modification 

 of epithelial cells, at the same time, they represent the simplest combination 

 of cells to form a mutually dependent sensory, nervous, and contractile 

 system. 



