370 THE PINEAL ORGAN 



G. H. Parker, in his description of the eyes of scorpions (1886), made 

 some important observations on their mode of development and of the 

 changes which take place in the relation of the fibres of the optic nerve 



pr. r 





hyp mb'?-- h yP- 



po.r 



Fig. 253. — Sections Illustrating the Development of the Median Eyes of 

 a Scorpion. (After G. H. Parker.) 



A — Right face of an approximately vertical section of a young embryo. The 

 lower part of the section is in the median plane ; the upper part is slightly 

 to the right of the median plane. The opening of the cavity has commenced 

 and is seen at the upper part of the fissure. The retina is inverted, its morpho- 

 logically superficial surface being situated on its deep aspect and directed 

 towards the brain, from which it is separated by the post-retinal layer. 



B C, D, E, F — Transverse sections at successively higher levels, through the 

 same region of a slightly older embryo, when the cavity has been fully opened 

 out. It shows the division of the stem of the Y-shaped cavity into two limbs, 

 which constitute the cavities of the right and left median eyes. It also shows 

 the emergence of the fibres of the optic nerve from the superficial aspect of 

 the retina. In the adult scorpion this relation is reversed. 



cav. : cavity. 



enc. : brain. 



fis. : lower end of cleft-like fissure 



beneath the fold forming the retina 



and pre-retina. 

 hyp. : hypodermis. 

 mb. : basement membrane. 

 mes. : mesoderm. 



in the transitional period between the larva and the fully developed 

 animal. He also made some interesting comparisons between the median 

 and the lateral eyes. He noted that the median eyes are situated close 



n.f. : nerve-fibres. 



nl. mes. : nucleus of mesoderm cell. 



n. opt. : optic nerve. 



po. r. : post-retina. 



pr. r. : pre-retina. 



r. : retina. 



sep. : septum. 



