Chapter 5 



THE GENESIS OF THE VERTEBRATE EYE 



(A) Embryological 



There are many anatomical relationships in the eye which are ex- 

 tremely puzzling when we look only at their adult condition, but which 

 become perfectly clear if we follow their ontogeny. A little knowledge 

 of the embryonic development of the eye is therefore highly desirable. 

 The process is a fascinating one in its own right, but we shall examine it 

 here as a means to two ends : the embryology of the eye can be expected 

 to shed some light upon its evolutionary origin; and, the developmental 

 scheme serves as a framework within which all possible adaptive evolu- 

 tionary changes of ocular structure must fit. If we know how the eye 

 develops we can guess where it came from, we can see how it has been 

 able to take on the modifications which fit it for greater efficiency in this 

 or that environment, and we can see why it has not been able to make 

 some changes that might seem to us more logical than particular ones 

 which it has happened to accomplish. 



The following account is a generalized one which applies in its en- 

 tirety to no particular animal, but is based upon the mammals because 

 their story is known in the greatest detail. Some important departures 

 characteristic of other vertebrate classes will be pointed out specifically, 

 but in general the reader who wishes to imagine the ocular embryology 

 of a lower class needs only to make a mental subtraction, from the mam- 

 malian process, of those features which the lower group lacks, in order 

 to have a fairly accurate conception. 



The parts of the eye are recruited from three sources in the embryo: 

 (a) the ectoderm of the neural tube, which is in turn derived by infold- 

 ing from the surface ectoderm and which later differentiates into the 

 brain and spinal cord; (b) the surface ectoderm remaining after the 

 neural tube has been formed and separated from it; and (c) the meso- 

 derm lying between the neural tube and the surface ectoderm. 



Formation of the Optic Cup — These three sources start to make 

 their respective contributions in this same order. The brain being by far 

 the most complex organ in the body, it begins to develop before any 

 other; and the eye gets an equally precocious start since its most essen- 



