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ESSENTIALS OF BIOLOGY 



epithelium (Fig. 28, 2), and obviously the sheet will increase in 

 thickness but will not spread out lengthwise. Clearly the sheet 

 will become warped, or curved, if the division-planes diverge 

 from the parallelism. 



Now suppose that only those cells in the region of the epithe- 

 lium marked as follows, x x x Xy divide by means of planes at right 

 angles to the surface (Fig. 28, 3) ; plainly the surface will become 

 arched, evaginated (4) or invaginated (5). Let the cell- divisions 

 cease at the summit of the evagination but proceed at the sides of 

 the latter (Fig. 28, 6), and it will easily be seen that the evagination 



Sclerotic 



Optic 

 nerve 



Fig. 29. — Diagrams showing the Development of the Vertebrate Eye. 



I, the structural plan of the eye; 2-8, stages in the development. 



will continue to grow out from the epithelium as a tube (6), and 

 if the divisions occur more rapidly on one side than on the other 

 (as at 7) the tube will be bent over. In this way evaginations 

 may take the forms of bent, twisted, branching, etc., tubes, bulbs, 

 etc. 



Now consider the mode of formation of such a very complex 

 organ as the vertebrate eye : the structural plan of the latter is as 

 indicated in Fig. 29. 



The formative stages in such an organogeny are these : At (2) 

 we see the epithelium of the fore-brain vesicle grow outward as 

 an evagination towards the integument of the side of the head. 

 This then becomes a bulb with a stalk (3) and the bulb enlarges, 

 the stalk at the same time becoming attenuated (4). Thus the 

 optic bulb is formed. The bulb then invaginates (5), becoming 



