76 BULLETIN OF THE 



of these figures, however (Fig. 9), there is some evidence that the row of 

 nuclei {tap.) is not single, but double, and that it is the result of an out- 

 folding of cells [tap.) lying between the regions pr. and r. This conclu- 

 sion is strengthened by the condition of the anterior lateral eye as shown 

 in Fig. 4, tap. It is almost certain, from the shape and direction of the 

 nuclei, that the equivalent region in this case is a fold, open helow. If 

 this middle region really represents a double rather than a single layer 

 of hypodermal cells, then the S-shaped appearance is deceptive ; and one 

 must suppose that half of the fold has become merged in one of the 

 thickenings (or otherwise obscured), while the other half remains as the 

 only apparent means of connection between the two thickenings. It is 

 further evident that this ow^folded middle region must be in the nature 

 of a re-entrant fold from the apex * of an original evolution, of which 

 the two thickenings constitute the walls. 



The condition and connections of this middle region are of great im- 

 portance in deciding upon the morphological relations of the retina, and 

 it is therefore to be regretted that the evidence as to its real nature is not 

 more conclusive. 



In the tract nearest the anterior median eye (Fig. 8, jo r.) the thickening 

 results simply from a displacement and a slight elongation of the cells 

 and their nuclei, the latter overlapping each other like so many tiles. But 

 the posterior thickening is more complicated ; it consists of two parts. 

 The anterior part is composed of cells, the nuclei of which have their 

 long axes nearly parallel with the surface of the head ; they collectively 

 form a broad band (r.) nearly perpendicular to the surface of the head ; 

 the nuclei are wedged between each other so as to form two or three 

 irregular rows. Behind this, and more or less in continuity with it, is a 

 region [pr r.) which gradually diminishes from a thickness nearly equal- 

 ling the length of the " perpendicular band," to the thickness of the or- 

 dinary hypodermis. The nuclei in this triangular region are, in the main, 

 perpendicular to the surface of the head, although showing a tendency to 

 radiate from a point near the deep end of the " band." There are, then, 

 four more or less distinct tracts already recognizable. These may be 

 named /ro»i behind forward, pre-^-etinal (pr r.), retinal (r.), tapetal (tap.), 

 BJid post-retinal (p r.), respectively. The same regions may also readily 



* It is possible that the re-entrant fold was not confined to the bottom (apex) 

 of the eye-pocket, but extended along its margins, and that the " fissure " in the 

 tapetum, subsequently referred to, is to be explained as resulting from the failure of 

 these two lateral ingrowths into the pocket to unite along the axis of the latter. 



