$8 Journal of Comparative Neurology. 



creases rapidly in size and soon assumes the form of a pear- 

 shaped vesicle. In the 12-13 n^m. stage it sends off two large 

 diverticula from its distal end. These in turn soon give rise to 

 secondary diverticula and cause the structure to appear digitate 

 in the later stages. Nerve fibres have not been observed and 

 it is probable that the structure is non-nervous. It does not 

 come in contact with the epiphyses up to the 30 mm. stage ; 

 beyond this we have not observed. 



Some General Considerations. 



The embryologist who contemplates the study of the pin- 

 eal organ soon recalls the old syllogistic statement : "The pineal 

 organ is probably a sense organ. The sense organs are paired 

 structures, ergo, the pineal organ should be a paired structure." 

 The «/r/6';7' reasons are undoubtedly cogent, and certain facts 

 and arguments have been adduced to prove their validity. Are 

 the facts adequate ? Are the arguments convincing ? 



It is quite beyond the province of the present paper to 

 to enter upon an extended review of the literature on the devel- 

 opment of the epiphysial structures in the different classes of 

 Vertebrata. It may not be premature, however, to glance over 

 the evidence thus far presented in the Fishes. 



A well defined parietal organ first appears in the Cyclo- 

 stomcs, yet there are suggestions of its forerunner in Amphi- 

 oxus. We quote the following from a paper by Dr. Ayers, 

 {Joum. MorpJi., Vol. IV, 1890, p. 228): "After a careful 

 study of the Amphioxus eye-spot and related structures I have 

 become convinced that the animal presents us with the earliest 

 stage in the phylogenetic development of the vertebrate eye." 

 Although this pigment presents a variety of forms the author 

 finds "The most usual form that of a slightly bilobed mass, 

 the lobes being placed to the right and left of the median line, 

 so as to cover the roots of the first pair of cranial nerves more 

 or less completely. " (p. 230). "For greater functional power, 

 the central (median) portion of the pigment spot has grown 

 upwards (dorsad) and carrying with it a portion of the ventric- 

 ular wall has produced the pineal eye." (p. 241). "The pari- 



