THE EYES. 61 



The Cerebral Grooves. — The cerebral ganglia gradually in- 

 crease in size (Plates VI, VII, figs. 14, 16 a, 19 a), and 

 their ventral surface becomes in Stage e markedly flattened 

 (PI. VII, fig. 23 a) and then invaginated (PI. II, fig. 35), so as 

 to form two grooves, which rapidly become narrower and deeper, 

 until in Stage f they form two slits, longitudinally arranged, 

 ending blindly in front, but opening behind into the buccal 

 cavity (Plate II, fig. 36, and Plate VIII, fig. 33). Eventually, 

 in old embryos of Stage r, they lose their external openings, 

 become reduced in size, and form two vesicles in the ventral 

 portions of the cerebral ganglia. Owing to the relatively greater 

 growth of the latter they appear to form, in the adult, small 

 hollow appendages of the brain (No. 20, PI. XVII, fig. D, d). 



The most important features which remain to be described 

 in the development of the preoral part of the central nervous 

 system, is the part which it takes in the formation of two 

 organs — the eyes and the tentacular nerves. 



The Eyes arise in Stage d as a pair of invaginations of the 

 postero-lateral parts of the cerebral rudiments (Plate II, fig. 29). 

 The two pits so formed are placed immediately in front of the 

 point of origin of the lip-folds from the pre-oral lobes (Plate II, 

 fig. 30). They are at first shallow, but soon become deeper, 

 and eventually (by the end of Stage e) constricted ofi" from the 

 surface, so as to form closed vesicles (Plates VI, VII, figs. 

 16 a, 15, 19 a, 22 a). The eyes, therefore, are nothing 

 more nor less than invaginations of the lateral por- 

 tions of the rudiments of the cerebral ganglia, and of 

 the surface ectoderm, which gave rise to and covers the latter. 

 Two elements enter into their composition, (a) the surface 

 ectoderm with its oval nuclei, and (b) the rounded nuclei of 

 the cerebral rudiment. The columnar surface nuclei form the 

 lining of the optic chamber, while the rounded elements which 

 lie behind the inner wall give rise to part of the retina and 

 optic nerve. Figs. 14, 15, 19 a, 22, show very clearly the 

 method of development of the optic rudiment. In the later 

 stages the connection between the posterior wall of the optic 

 vesicle and the cerebral ganglion becomes somewhat con- 



