COORDINATION IN ANIMALS 199 



VESICLES arise as lateral outpocketings directly from the fore- 

 brain. (Fig. 111.) 



A retina alone such as exists in some of the lower Inverte- 

 brates can afford no visual sensations other than light and 

 darkness, and perhaps in some cases the ability to distinguish 

 light of one color from that of another. In order that not 

 merely degrees of the intensity of light may be perceived, but 

 that objects may be seen, many of the higher Invertebrates 

 have developed various kinds of complicated apparatus for 

 bringing the rays from a given point to a focus at one point 

 on the retina, culminating on the one hand in the mosaic 

 vision of the Arthropods, and on the other hand in the camera 

 eye of the Cuttlefish. In the latter case the mechanism is 

 quite similar to that found in the Vertebrates, but since it 

 occurs in the group of Molluscs which cannot be considered 

 in the direct evolutionary line of the Vertebrates, it affords 

 an example of similar responses of different organisms to 

 similar needs giving rise to analogous structures. (Fig. 112.) 



In the development of the Vertebrate eye, the hollow out- 

 growth or optic vesicle (one of which arises from either side 

 of the diencephalon) gradually extends toward the outer sur- 

 face of the head, where it becomes associated with an in- 

 pocketing of the ectoderm. The latter gradually becomes 

 separated from the surface ectoderm as a sac, the very thick 

 walls of which almost completely obliterate its cavity. This 

 sac is destined to become the LENS, and as it enlarges it comes 

 in contact with the optic vesicle, which now is connected 

 with the point of origin from the diencephalon by a narrow 

 isthmus (OPTIC STALK). Apparently under the influence of 

 the developing lens, the optic vesicle is invaginated and there- 

 by transformed from a single-layered structure into a double- 

 layered cup (OPTIC CUP). These two layers form the retina, 

 the inner layer becoming differentiated into the essential 



