TYPES OF VERTEBRATE AND INVERTEBRATE EYES 39 



inherited condition. Moreover, that multiple growth-centres, twin and 

 single monsters may occur as the result of alterations in the environmental 

 conditions has been irrefutably demonstrated by experimental methods 

 in the lower types of animals, and their occurrence appears to be extremely 

 probable as a result of injury, disease, or toxic conditions of the blood 

 circulating in the placenta in mammalia. 



These conclusions, as we shall see hereafter, have a very definite 

 bearing on the development and inheritance of the single median-eyes of 

 both invertebrates and vertebrates, and their study with special reference 

 to cyclopia is, therefore, not irrelevant to the general purpose of this 

 book. Both types of cylopia, namely, that occurring in double-headed 

 monsters and that arising from a defective growth of the interocular 

 region in single monsters, are brought about by the growth in contact 

 with each other of apposed parts of two eyes, while the development of the 

 remaining parts of the eyes which are situated in the interval between them 

 is suppressed. The principle which is involved is the same whether the 

 cyclops eye is developed from the outer or temporal segments of two eyes 

 growing in apposition with each other, as in one of the two secondary 

 faces of a Janus-headed monster, or whether the cyclops eye is formed from 

 the inner or nasal segments of two apposed eyes, as in the double-faced 

 (Diprosopus) monster, in which the two faces look in the same direction 

 and all eyes are to the front ; moreover, it is evident that, whatever other 

 causes may be at work, this same principle, of the suppression of the growth 

 of the intervening parts, is concerned in the development of a cyclops eye 

 in single monsters. 



The following questions arise in connection with the causes of the 

 suppression or arrest of development ; namely, is it due to : 



1. A lack of building material from which to form the missing 



parts ? 



2. A lack of or defect in the organizing power of a growth centre ? 



3 . A displacement or arrest of development due to the growth of two 



parts in apposition with each other ? 

 Finally, is the arrest to be considered as due to a combination of any two 

 or all of these three factors ? 



Before attempting to answer these propositions we will pass on to the 

 general question of the production of double-monsters and give a brief 

 historical sketch of the earlier conceptions of the problems which are 

 involved, one of which held that a double monster was the resu't of a 

 splitting of the early embryonic rudiment, either at the head-end — 

 " anterior dichotomy " — or at the tail-end — " posterior dichotomy " ; 

 the other that two embryonic rudiments appeared in a single embryonic 

 area and that these rudiments afterwards came into contact and grew 



