No. 2.] DEVELOPMEN'T OF PETROMYZOIsr. 259 



teenth day as a small median diverticulum of the roof of the 

 fore-brain, and until after the time of hatching the communica- 

 tion between the cavity of the diverticulum and the third ven- 

 tricle remains open. This communication is, however, shut off 

 shortly after hatching, though a narrow prolongation of the 

 ventricle long persists as a recessus pinealis. At first the 

 pineal rudiment is composed of a layer of columnar epithelial 

 cells in no way differing from the surrounding cells in the roof 

 of the brain. In a short time, however, the cells forming the 

 lower wall of the diverticulum become perceptibly thicker, a 

 result which is produced not by division, but by vertical elonga- 

 tion of the cells and gradual assumption of a more or less rod- 

 like form. The epiphysis rapidly increases in size, and grows 

 especially forwards, so that the distal vesicle comes to overlie and 

 even extend beyond the ganglia habenulae, whereas, at its first 

 formation, it lies altogether behind the region of these bodies. At 

 this stage, and for some time afterwards, the vesicle is much com- 

 pressed by the surrounding structures, so that its cavity is nearly 

 obliterated and its structure very much obscured. When viewed 

 with low powers, the body appears to consist of a flattened, 

 lenticular, and solid mass of rounded cells (Figs. 5, 6, and 9, PI. 

 VIII; 12, PI. IX) ; but a closer examination of more favorable 

 sections reveals a small cavity with a thin roof of much flat- 

 tened cells and a thickened floor of columnar cells. It is to be 

 observed that the portion of the vesicle which gives rise to the 

 retina-like structure is the inner wall, that is to say, the wall 

 turned toward the brain, while in the paired eyes the retina 

 arises in the side of the vesicle which is turned away from the 

 brain and nearest to the skin. Thus not only is the position of 

 the retinal elements with reference to the nerve fibres different 

 in the pineal and the paired eyes, but the portion of the primi- 

 tive vesicle which becomes the retina is also different. Or, if 

 the retinal side be considered homologous in the two sets of 

 organs, the vesicle has changed its position with reference to 

 the brain and the skin. Beard (7 and 8) states that the epi- 

 physis is pigmented in some specimens of the adult and in the 

 very young larvae. Though not wishing in any way to throw 

 doubt upon this positive statement of fact, I am unable to con- 

 firm it, as none of my sections of Ammoccetes show any trace 

 of pigmentation. The presence of pigment is, of course, an 



