128 THE GENESIS OF THE VERTEBRATE EYE 



shown by Benoit and others to act as a photic receptor organ, controlling 

 reflexly the annual spermatogenetic cycle. 



But Studnicka never considered in detail the manner in which rods 

 and cones differentiate, though this had already been most carefully 

 worked out by several European investigators. If he had done so, his 

 theory would surely have seemed much stronger to subsequent text- 

 writers. For the outer segment, the receptive organelle, of a vertebrate 

 visual cell develops exactly like any flagellum (Fig. 55a, b). It starts as 

 a filament of centrosomic material rooted in a diplosome or dumb-bell 

 shaped centriole embedded in the future inner segment, later becoming 

 encrusted and thickened by mitochondria which form the conspicuous 

 spiral filaments making up the bulk of the outer segment (Figs. 23a, 

 25c, pp. 55, 62; Fig. 26b, B, p. 63). A closer comparabiUty of visual cells 

 and ependymal cells (Fig. 55c, d) could hardly exist. 



Origin of the Retina — If the photosensory parts of the rods and cones 

 were once ependymal flagella, it is certain that Boveri's theory must be 

 discarded; for ependyma, even photosensory ependyma, exists in Am- 

 phioxus side by side with the Hesse's organs and Joseph's cells. It is 

 equally certain that the vertebrate retina could not have gotten started, 

 as a photosensitive region of the brain wall, until the latter had become 

 tubular. Only then was there any need for the ependymal cells to evolve 

 as elements distinct from nerve cells; and these were primarily supportive 

 (they still run through the whole thickness of the brain wall in Am- 

 phioxus and the lampreys), then secretory in function (producing the 

 cerebrospinal fluid) before it became necessary for them to aid in circu- 

 lation by means of flagella. No flagella, no sensitivity or photosensitivity; 

 and it can be regarded as certain that the definitive visual cells were 

 developed within the finished brain and not, a la Balfour, while the nerv- 

 ous system was still a part of the skin. Indubitably there were photo- 

 irritable cells in the provertebrate's skin, as there still are in many fishes 

 and amphibians — even in cave forms which are never normally struck by 

 light; but these lost importance as soon as photosensory ependyma had 

 appeared (Fig. 56). The most primitive homologues of the rods and 

 cones to which we can point today are the photosensory flagellated epen- 

 dymal cells of the 'infundibular organ' of Amphioxus (Fig. 45, inf, p. 

 120), which is a crude visual apparatus seemingly for the detection of 

 the direction of light by means of shadows cast upon it by the anterior 

 pigment spot. 



