TYPES OF VERTEBRATE AND INVERTEBRATE EYES 35 



eyes which have grown in contact with each other have been suppressed, 

 or, more correctly, have failed to develop. In Cephalo-thoracopagus 

 monosymmetros it is the inner parts of the developing facial regions which 

 first come into contact with each other on the cyclopic side, and these 

 consequently fail to develop, whereas the outer parts of the face, being 

 deflected, remain free, and thus undergo development ; this will be 

 explained later when dealing with the regulation of growth by the 

 influence of an organizer, namely the dorsal lip of the blastopore, as 

 when the normal course of development is interfered with by growth 

 contact, in the experimental grafting of two gastrula-halves together 

 and the formation of a Duplicitas cruciata, as described by Spemann 

 (Figs. 26 and 27). 



In the second type of cyclops, namely that occurring in single-headed 

 monsters (Figs. 28, 29, and 30), the cyclops eye is formed by the more or 

 less complete fusion of the outer or temporal segments of the right and 

 left eyes of a single individual. It is accompanied by absence or defective 

 growth of the median parts of the head, including the inner or nasal seg- 

 ments of the eyes and eyelids, the nose and mouth, the frontal region of 

 the cranial part of the skull, and the corresponding parts of the brain. 

 It is common in domestic animals — cats, dogs, sheep (Fig. 28), and cattle 

 (Fig. 28 A, b) — and frequently occurs in artificially reared fish, amphibian 

 and reptilian larvae, and also in artificially incubated eggs. The injurious 

 agents which have been employed in the experimental rearing or the arti- 

 ficial incubation that have resulted in the formation of these anomalies are : 

 (1) mechanical injuries, e.g. separation of the blastomeres, incision, excision, 

 and destruction of growing parts ; (2) the addition of magnesium, lithium, 

 and other salts to the water in which the larvae are reared ; (3) raising or 

 lowering the temperature above or below the normal ; (4) variations in 

 the amount of oxygen or carbon dioxide ; (5) narcotics and poisonous 

 drugs such as chloroform, chloretone, and potassium cyanide. The 

 results obtained by such treatment vary not only with the degree of the 

 injury inflicted and the part injured, but also with the particular stage 

 of development reached at the time when the experiment is made. Thus 

 the production of normal or of cyclops eyes in Triton or Ambly stoma 

 larvae appears to depend on the presence or absence of the anterior part 

 of the entodermal gut-roof, which acts as a regulator or organizer on the 

 presumptive eye-region in the neurula stage of development. If the 

 anterior and central region of the entodermal gut-roof is injured there is 

 a liability to cyclops formation, owing to interference with the normal 

 organization of the optic vesicles and cerebral hemispheres from the over- 

 lying neural ectoderm. If the whole eye-region is destroyed, 

 anophthalmia will result. 



