The Brain of Diemydyhis Vitidescens 273 



Membranes. — The pia is a very delicate membrane, in many- 

 places clinging closely to the dura. Its relations are the same, 

 as far as traced, as in the adult, it supplies comparatively very 

 few blood vessels to the brain ; a few in the olfactory region 

 (Fig. 75) ; in the region between the mesen- and epencephal 

 (Fig. 82) ; and to the medulla (Fig. 85). Contrast the capil- 

 laries of Plates III and IV with VII. 



The arachnoid is represented by a few connective tissue 

 cells, and can be clearly seen only in the cephalic region in 

 the triangular spaces between pia and dura (Fig. 75). 



The dura is thin, and in the dorsal part where the cartilage 

 of the skull is not developed, lies in close contact with the epi- 

 dermis. In this part a few pigment cells exist. 



The endolymphatic sacs are slightly developed and appear 

 in only a few sections (Fig. 83, 84), they are closely applied to 

 the lateral recesses of the epicoele but do not extend over the 

 metaplexus. The canal connects it with the ear more directly 

 than in the adult. 



Nerves. — The methods, as stated, are not especially adapted 

 to the study of nerve tracts, hence little beyond the external 

 origin is mentioned. The relations in the larvae are often 

 clearer than in the adult. 



I. The olfactory nerve in both young and adult has two 

 lateral roots ; in the latter a whorl-like arrangement of cells 

 is seen in the lobe and more proximal part of the roots, that 

 is not present in the larva. The roots unite to form one 

 trunk which divides, sending branches to surround the nasal 

 epithelium and to Jacobson's organ. These observations 

 agree with Burckhardt (6) rather than with Kingsley (30), 

 who found only one root in larval amphiuma. 



II. The optic nerve is similar in larva and adult ; the 

 central cells of the larva occupy a much larger proportion of 

 the nerve. In both, at the exit from the brain there are no 

 fibers on the dorsal side of the nerve (Fig. 20) ; the fibers 

 twist dorsad and soon form a complete tubular investment for 

 the cells. The chiasma is well developed. 



III. As shown in figures 23, 44, 38, 81, this has the usual 

 place of origin. In the larva the commissure across the meson 

 is cut off from other alba by cinerea (Fig. 67). 



