70 WITHIN AND WITHOUT 



wonderful, but by no means so wonderful as what 

 happens in other cases in which the entire lens 

 has been removed and the new lens grows from the 

 outer pigmented layer of the margin of the iris. 

 To the unbiological reader one source of origin 

 will not seem more wonderful than the other, 

 but there is really a vast distinction between them. 

 At an early stage in the development of the em- 

 bryo, the cells composing it become divisible into 

 three layers. It is even possible, as Loeb main- 

 tains, that this differentiation is present in the 

 unsegmented ovum, in which case the facts to 

 be detailed become still more remarkable and 

 significant. These layers are known as epi-, 

 meso-, and hypo-blast ; and from each one of 

 them arise certain portions of the body, and 

 certain portions only. It would be as remarkable 

 to a biologist to find these layers not breeding 

 true as it would to a fowl-fancier to discover that 

 the eggs of his Buff Orpingtons were producing 

 young turkeys or ducks. Now the lens is an 

 epiblastic structure, and the iris is mesoblastic. 

 Hence the wonder with which we are filled when 

 we find the iris growing a lens. Loeb attempts 

 to explain this in the first instance by telling us 

 that the cells of the iris cannot grow and develop 

 as long as they are pigmented ; that the operation 

 wounds the iris, allows pigment to escape, and 

 thus permits of proliferation. We may accept 

 this, and yet ask why it takes on a form of growth 

 familiar to us only in connection with epiblast ? 

 The reply is : " Young cells when put into the 

 optic cup always become transparent, no matter 



