212 W. BALDWIN SPENCEE. 



logically, the structure of the retina could not be determined 

 with any great amount of accuracy. The rods as usual formed 

 the most prominent feature ; at their external ends in certain 

 parts spherical elements could be distinguished (n^), whilst, most 

 externally, elements corresponding doubtless to the cone-shaped 

 ones of other eyes were present (Co.). In many parts external 

 to the rods masses of pigment (pig-^), indicating doubtless 

 degeneracy in the tissues of the retina, were present. 



Connection with the brain. — The eye is apparently 

 completely separated off from the brain, no pineal stalk being 

 recognisable. 



General Account of the Structure in Lacertilia. 



The above account reveals the epiphysis within the group 

 Lacertilia as a structure of very varied development, in 

 some forms presenting merely the appearance of a hollow 

 process from the roof of the thalamencephalon, in others being 

 modified into a well-marked eye, whilst between these two 

 extremes various intermediate forms are found. In taking a 

 short general review of the results detailed above we may deal 

 with them under the four following heads : 



(1) General Form of the Epiphysis.^ — The simplest form 

 seen is in Platydactylus, where it has merely the structure of 

 a hollow outgrowth running at right angles to the surface of the 

 thalamencephalon until it reaches the dura mater lining the 

 cranial cavity. In H atteria, on the other hand, we have a form 

 in which specialisation is carried to its furthest extent, with the 

 result that the epiphysis becomes modified into three parts — (1) 

 a proximal part, still hollow, and connected with the brain roof, 



(2) a median, solid pineal stalk, serving to connect the former with 



(3) the distal portion differentiated into an optic organ. These 

 forms may be taken as two extremes, the gap between which is 

 filled up by various modifications : thus in Cyclodus the epi- 

 physis instead of running straight upwards turns forwards, and at 

 the distal end swells out into a vesicle whose walls show a trace 



' Compare the diagram showing the development of the epiphysis in various 

 forms on PI. XX. 



