THE BRAIN. 123 



to the right and left of this the sides become thickened by the 

 formation of the crura cerebri, two longitudinal bundles of nerve 

 fibres which connect the mid-brain with the fore-brain. The 

 roof of the mid-brain is thin in the early stages ; but shortly 

 after the opening of the mouth the two halves of the roof thicken 

 considerably, and, bulging upwards, form a pair of rounded swell- 

 ings, the optic lobes (Fig. 62, ol), separated by a median groove. 

 The optic lobes continue to increase in size, and about the 

 time of the metamorphosis become, as in the adult frog, the 

 widest portion of the brain. The cavity of the mid-brain 

 persists as a fairly wide passage, the Sylvian aqueduct. 



I The thalamencephalon is the original fore-brain of the 

 embryo; and in connection with it important changes occur. 

 Its cavity, the third ventricle, is at first large ; but, owing to 

 thickening of its walls to form the optic thalami, the cavity 

 becomes early reduced to a vertical cleft, very narrow from side 

 to side. 



The roof of the thalamencephalon is very thin, consisting, 

 like that of the medulla oblongata, of a single layer of epithelial 

 cells, devoid of nervous elements. About the middle of its 

 length, and at the place where the final closure of the neural 

 tube was effected, the pineal body is formed. This appears 

 in embryos of about 3 mm. length (Fig. 61, PN) as a median 

 hollow diverticulum, which at the time of hatching of the 

 tadpole (Fig. 64, rx), forms a small round knob on the top of 

 the brain, immediately beneath the surface epiblast. This 

 grows forwards, and becomes dilated distally. At the time of 

 opening of the mouth it forms a small rounded vesicle, connected 

 with the brain by a tubular stalk ; it is of a glistening white 

 appearance, owing to the presence of small snow-white particles 

 imbedded in its substance, and stands in this respect in marked 

 contrast to the rest of the brain, which is pigmented rather 

 strongly. In tadpoles of 12 mm. length the pineal body itself 

 is solid (Fig. 65, ris T ), but its stalk is still tubular. Shortly 

 after this, on the formation of the skull, the pineal body becomes 

 cut off from its stalk, and lies outside the skull, just beneath 

 the skin of the top of the head. It persists throughout the 

 tadpole stages, but disappears at the time of the metamorphosis. 

 The stalk of the pineal body persists throughout the life of the 



