356 ARTHUR E. SHIPLEY. 



on its ventral side ; by the twenty-second day this optic 

 chiasma is covered in by a single layer of ganglion cells. It is 

 this body that Dohrn has by mistake figured as the Tuber 

 cinereum (21). The commissure is shown in transverse 

 section in fig. 39 ; the lumen of the infundibulum is seen 

 below it, the cavity of the fore-brain above. 



About the same time that the optic vesicles commence to be 

 given off from the anterior end of the brain a median dorsal 

 evagination also appears. It was mentioned above that in the 

 median line, both dorsally, ventrally, and in front, the central 

 canal is enclosed by a single layer of more or less columnar 

 cells, whilst the lateral walls are thick. This single layer is 

 interrupted ventrally by the formation of the optic chiasma. 

 Dorsally it is produced on the sixteenth day by the evagina- 

 tion in question, which is the rudiment of the pineal gland 

 (fig. 31). The walls of the pineal gland then consist at first 

 of a single layer of cells forming a hollow sac which pushes its 

 way between the brain and the epidermis, spreading out on all 

 sides (fig. 31). At first its lumen is continuous with that of 

 the fore-brain, but ultimately, by the folding of its walls, its 

 cavity is obliterated and the commuuication with the lumen of 

 the fore-brain is shut off. 



The eighteenth day, two days after the first appearance of 

 the optic vesicles and the pineal gland, is the earliest date 

 on which I have been able to recognise the appearance of any 

 division into fore-, mid-, and hind brain. On this day the 

 single layer of cells roofing the central canal becomes folded in 

 the manner indicated in fig. 23. This takes place at about 

 the level of the attachment of the velum, a little in front of 

 the ear. In larva of fifty-two days, this groove has not 

 changed its form, but has become deeper. 



The division between the fore- and hind-brain is by no 

 means so well marked ; indeed, I have been unable to find any 

 external groove, although it has been described by previous 

 writers. Longitudinal horizontal sections through the brain 

 show, however, that just behind the infundibulum and pineal 

 gland the walls thin out so that the lumen appears diamond 



