20 Journal of Comparative Neurology. 



strong epithelium layer .04-05 mm. The cylindrical cells of 

 the follicle are .03 mm. high and the round cells .007-. 008 mm. 



Development of the pineal in the rabbit. The processus pine- 

 alis in the rabbit is developed as in the bird although in a rela- 

 tively more hightly developed stage. The brain of a chick of 

 4^ days, corresponds to the embryonic rabbit 12-13 mm. l° n g> 

 although the first trace of a projection was found in an embryo 

 15—16 mm. long. The projection is considerably shorter 

 (.15 mm.) and therefore is not bent so obliquely forward as 

 in birds. It lies wedged in between the blood vessels of the 

 corpus callosum. The constriction of the diverticles occurs in 

 embryos 2 to 2.5 cm. long. The vesicles are as large as in the 

 chick but their cavities are very small, hence the latter easily 

 escape notice. The cavity then disappears entirely and all the 

 cells become round or polygonal and are provided with shorter 

 projections. 



In embryos 3.5 to 4 cm. long the gland is pear shaped, 

 its ventral aspect being in connection with the recessus infrapin- 

 ealis while the dorsal end is closely united with the surface of 

 the hemispheres. Later through the backward growth of the 

 brain it becomes covered and lies under the corpus callosum in 

 the fiss. transv. cerebri. The gland retains its connection with 

 the diencephalon by remaining in connection with the post cere- 

 bral commissure, with the taeniae thalami by a nervous connec- 

 tion, on either side, and with the pedunculi conarii. 



Histological stmctnre. As to its histological structure the 

 pineal of mammals in its embryological relations has suffered a 

 significant deviation, in that the epithelium follicles have thrown 

 off the vesicular form and the cylindrical cells have been trans- 

 formed into round and polygonal cells. The development 

 shows however that these cells are not lymph bodies. 

 Neither can they be called nervous, first because they lack the 

 necessary characteristics, and second because they are never 

 found in connection with nerve fibres. As to their origin they 

 are homologous with the epithelium cells of the ventricle and 

 vascular plexus, and have undergone a metamorphosis similar 

 to that of the epithelium cells of the enamel organ. Hence the 



