BY W. J, McKAY. 877" 



External appearance of the pineal eye in the embryos. — In the 

 embryos of Hinidia, which were advanced in development, the 

 pineal eye could be seen standing out as a projection, at a point 

 where the anteiior joined the middle cerebral vesicle (PL xxiii., 

 figs. 7, 7a, Pn.). The projection viewed as a solid object appeared to 

 be composed of two lobes, an anterior larger, and a posterior smaller 

 one. This appearance was explained when a longitudinal section of 

 the head had been made ; the anterior lobe was seen to be the pineal 

 eye proper, while the posterior lobe was the curved end of the 

 epiphysis (fig. 7, Pn. Ejj.). En the more advanced stages of 

 Hinulia, the eye appeared as a black spot in the median line of 

 the head posterior to the paired eyes. 



I have not been able to work out the earliest stages in the 

 development of the eye in Hinulia, but I have obtained sufficient 

 specimens of Grammatojyhora to give a fairly complete history of 

 its earliest stages ; and where the stages in this form ended, I 

 have been enabled to complete them in Hinulia. 



Gkammatophora muricata. 



First Stage. — On making a vertical longitudinal section of the 

 head of an embryo of this species, the walls of the cerebral vesicles 

 are seen to be composed of columnar cells, covered by a layer of 

 epiblast. The epiphysis cerebri or pineal gland arises, as is seen, 

 as an outgrowth of the thalamencephalon. At this stage the 

 outgrowth is composed of a single layer of columnar cells with 

 well-marked nuclei (fig. 1). 



Second Stage. — In the next stage the evagination or vesicle 

 undergoes the following changes. The anterior wall begins to gi'ow 

 forward, and this soon leads to theformatioQ of a second evagina- 

 tion in the wall of the primary one (fig. 2, Pn.). Thus- 

 we have two vesicles formed, an anterior larger {Pn.), destined 

 to become the pineal eye, and a posterior smaller one {Ep.). 

 Since the anterior vesicle grows faster than the posterior, it bends 

 forwards, and its inferior wall rests on the superior surface of the 



