SUMMARIAL ACCOUNT OF THE EYE OF THE AMBLYOPSID/E. 181 



is undoubted degeneration, it is always an indication that the eye has not yet 

 reached the point where it is in equilibrium with its functional requirements. 



Cessation of development does not take place at the same time in all parts of 

 the eye. Those not essential to the perception of light are disturbed first. The 

 retina and the optic nerve are the last affected, the iris coming next in the series. 

 Because the cornea, aqueous and vitreous bodies, and the lens are not essential 

 for the performance of the function of the eye, these structures cease to develop 

 early. The processes of degeneration follow the same rate. Degeneration is 

 brought about by the falling apart of the elements as the result of the introduction 

 of connective tissue cells that act as wedges. Abnormal degeneration sometimes 

 becomes manifest through the cessation of the reduction of parts that normally 

 decrease in size so that these parts in the degenerate organ are unusually large. 



Kohl's theoretical explanation here given somewhat at length is based on the 

 study of an extensive series of degenerate eyes. He has not been able to test the 

 theory in a series of animals living actually in the condition he supposes for them, 

 and has permitted his erroneous interpretation of the highly degenerate eye of 

 Troglichthys to lead him to this theory of the arresting of the eye in ever earlier 

 stages of ontogeny. It has been shown in previous pages that this most degenerate 

 eye is in an entirely different condition from that supposed by him. The mere 

 checking of the normal morphogenic development has done absolutely nothing 

 to bring about this condition, and it could not have been produced by the checking 

 of development in ever earlier and earlier stages of ontogeny, for there is no stage 

 in normal ontogeny resembling in the remotest degree the eye of Troglichthys. 

 The process of degeneration as seen in the Amblyopsidae is in the first instance 

 one of growing smaller and simpler not a cutting off of late stages in the develop- 

 ment. The simplified condition, it is true, appears earlier and earlier in ontogeny 

 till it appears almost along the entire line of development, even in the earliest stages. 

 But the tendency for characters added at the end of ontogeny to appear earlier and 

 earlier in the ontogeny is well known, and there is no inherent reason why an organ 

 disappearing in the adult should not eventually disappear entirely from ontogeny. 

 The fact that organs which have disappeared in the adult have in many instances 

 not also disappeared in the ontogeny and remain as so-called rudimentary organs 

 has received an explanation from Sedgwick. For a discussion of this see the chap- 

 ter on the Law of Biogenesis. 



In Amblyopsis, where the eye has not been functional at any period of ontogeny 

 for many generations, where degeneration begins at an early period and continues 

 till death, the degenerate condition has reached the early stages of the embryo. 

 It is only during the first hour or so that the eye gives promise of becoming any- 

 thing more than it eventually does become. The degree of degeneration of an organ 

 can be measured as readily by the stage of ontogeny when the degeneration becomes 

 noticeable as by the structure in the adult. The greater the degeneration, the farther 

 back in the ontogeny the degenerate condition becomes apparent, unless, as stated 

 above, the organ is of use at some time in ontogeny. It is evident that an organ 

 in the process of being perfected by selection may be crowded into the early stages 

 of ontogeny by post-selection. Evidently the degenerate condition is not crowded 

 back for the same reason. How it is crowded back, I am unable to say. A satis- 

 factory explanation of this will also be a satisfactory explanation of the process 



