CHAPTER IX. 



THE COMPOUND EYES OF ARTHROPODS AND THE LATERAL 

 EYES OF VERTEBRATES. 



Froriep (Hertwig's Handbook of Embryology) states, quoting Kessler, 1877, 

 that K. E. von Baer's discovery that the eye in the chick is a hollow outgrowth of 

 the forebrain vesicle, is the most interesting fact in the development of the eye 

 that could have been obtained, and is without a parallel. He also quotes with 

 approval Gegenbaur's expression of astonishment that in the entire range of 

 vertebrates there are no lower stages in the development of such a complex organ 

 as the eye. The vertebrate eye, he says, Athene-like, makes its appearance com- 

 pletely formed, and comparative anatomy and embryology are powerless against it. 



The problem, however, is not as hopeless as this, for we have shown that the 

 parietal eye of arachnids furnishes a very striking parallel to the development of a 

 vertebrate cerebral eye. The arachnid theory also provides a satisfactory ex- 

 planation for the sudden appearance of lateral, cerebral eyes in vertebrates, and for 

 their most striking peculiarities. It is clear, on the arachnid theory, that the lateral 

 eye was delivered to the vertebrates in a high stage of perfection. There are no 

 transitional stages between the external, convex eye of arthropods and the internal, 

 concave eye of vertebrates, because there can be no half-way stages between an 

 eye that stays outside the brain chamber, and one that during development is 

 carried into the chamber. The eye must either get in early, before the brain 

 closes, or stay out. Either position, at once and definitely, determines its char- 

 acter and the way it does its work. Whatever moulding influence the new en- 

 vironment had on the enclosed eye was felt immediately, and the necessary read- 

 justments, no doubt, at first followed rapidly and then ceased, leaving the eye 

 more stable than before, because enclosed in less variable surroundings. 



I. Compound Eyes of Arthropods. 



A. Serial Location. — The lateral eyes of arthropods are such essential and 

 constant parts of the head that it is important to determine their origin, and to 

 what metamere they belong. This, however, is a very difficult thing to do. 



The view, often expressed, that the compound eyes are compact groups of 

 larval ocelli is untenable, since the primitive larval ocelli (coleoptera and lepi- 

 doptera) degenerate and take no part in the formation of the lateral eyes. More 

 over, in the very early larval stages of phyllopods, copepods, and many other 

 Crustacea the larval ocelli and compound eyes are present at the same time and 

 clearly arise independently of each other. 



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