178 organisers: inducers of differentiation 



properties of the tissues acted upon : the various grafts mentioned 

 in the previous paragraphs do not override the local regional 

 potencies, but merely evoke them (hg. 85). 



We may now return to cases in which the dependent differentia- 

 tion appears definitely to be due to chemical effects arising from 

 proximity with some other organ. A classical example is the de- 

 pendence of the conjunctiva^ upon the presence of the eye. 



In the absence of contact with an optic vesicle, the epidermis of 

 the presumptive conjunctiva region remains pigmented and opaque. 

 If, however, contact is established, it loses its pigment and becomes 

 transparent.^ This effect is exerted not only by the whole optic 

 vesicle, but also by portions of the retina, by the lens alone, and 

 even by disorganised fragments of the optic vesicle grafted under 

 the skin. It even appears that an engrafted limb occupying the place 

 of an eye is capable of inducing the differentiation of the conjunctiva.^ 

 Pieces of already differentiated epidermis from other regions 

 grafted over the eye, or when eye or lens is grafted under them, 

 can be induced to undergo modification into conjunctiva.* 



A case which is in many ways comparable with that of the con- 

 junctiva is provided by the Anuran tympanic membrane. This 

 structure is differentiated out of the epidermis at metamorphosis 

 by means of processes involving histolysis and reconstructions of 

 certain layers. Here, the annular tympanic cartilage is the structure 

 on which the differentiation is dependent. Epidermis from other 

 regions of the body will differentiate into tympanic membrane if 

 grafted over the tympanic cartilage, and if the cartilage is extir- 

 pated, no membrane forms. Tympanic cartilage grafted under the 

 skin of the back induces the formation of a tympanic membrane 

 in that place.^ A similar case is the differentiation of an articular 

 cup on the palato-quadrate to fit the base of the balancer. This is 

 dependent on the presence of the balancer, and its formation can 



^ The conjunctiva is of course the epidermal, and the cornea the mesodermal 

 layer of tissue overlying the pupil and lens. These terms have often been used 

 very carelessly, the conjunctiva being called the cornea, and vice versa. In most 

 cases, the experiments have not been carried on long enough for the cornea to 

 become properly differentiated. 



2 Spemann, 1901 a; Lewis, 1905. 



^ Diirken, 1916. 



^ Fischel, 1917; W. H. Cole, 1922; Groll, 1924, 



^ Helff, 19^8. 



