72 ARTHUR DENDY. 



simple, backward-pointing diverticuluin of the roof of the fore- 

 brain some considerable distance in front of the parietal eye. 

 At Stage O the cerebral hemispheres and optic lobes are both 

 prominent externally, with the thin roof of the thalamence- 

 phalon stretched out between them. Just in front of the optic 

 lobes lie the parietal eye and ''stalk," and just behind the 

 cerebral hemispheres lies the pineal gland, with its wall just 

 beginning to be folded. At Stage R the cranial flexure has 

 become straightened out, and the cerebral hemispheres and 

 mid-brain much more closely approximated to one another, so 

 that the roof of the thalamencephalon has become shortened 

 and folded, giving rise to the choroid plexus ; while the para- 

 physis and parietal eye, which develop quite independently of 

 one another and are never really connected, are brought close 

 together. The paraphysis has now given rise to a convoluted 

 mass of tubules, intermingled with blood-vessels, which comes 

 to lie beneath the parietal eye and in front of its so-called 

 *' stalk. ^^ Meanwhile the superior commissure has made its 

 appearance just in front of the point where the stalk of the 

 parietal eye joins the brain. The details of the development 

 of these organs, however, are beyond the scope of the present 

 paper, and will be dealt with in a later memoir. 



The Ordinary Paired Eyes. — The development of the 

 paired eyes, so far as investigated, is shown in fig. Ill , from 

 which it will be seen that it takes place in the normal verte- 

 brate fashion. The optical vesicle is invaginated to form an 

 optic cup, and the lens is formed as a thickening of the super- 

 ficial epiblast opposite to the mouth of the cup. 



The Parietal Eye. — The development of this organ will 

 form the principal subject of the special memoir already referred 

 to, but it may be worth while to summarise the principal facts 

 in this place. It is formed from the primary parietal vesicle, 

 the origin of which as an outgrowth from the roof of the fore- 

 brain slightly on the left of the middle line has been noted. 

 The front part of the wall of this vesicle thickens to form the 

 lens, while the deeper part gives rise to the retina. The latter 

 forms a deep optic cup which holds ihe lens in its mouth, the 



