The Brain of Diemydylus Viridescens 311 



light!}' shaded portion of the geminum {gm.) which approaches the 

 meson ; the small cerebellum ; the simple metaplexus ; and the ap- 

 proximation of the hypophysis and notochord. The space between the 

 medulla and pia may not be natural. The numbers refer to the cor- 

 responding figures of other plates. 



Fig. 68. A more nearly mesal section of the same series as Fig. 66 to 

 show the persistent opening of the epiphysis and its relation to the 

 commissures. X 120. The habena is seen in face view. 



Fig. 69. Frontal section of the same series as Fig. 2 at the level 

 shown in Fig. 67. Shows the two roots of the olfactory nerve ; the 

 undiflFerentiated form of the rhino- and paracceles ; the caudal expan- 

 sion of the mesoccele. 



Fig. 70. An enlargement of Fig. 69 to show the paraphysis and dia- 

 plexus. X 120. 



Fig. 71. A section ventrad of Fig. 69. At ec ih.^ mesal cells are at 

 a more ventral level. Shows the cephalic extension of the aula. 



Fig. 72. An enlargement of the crista of Fig. 71. X 120. 



Fig. 73. A transection through the head of an embryo of 12 days, to 

 show the cephalic flexure and the position of the paraphysis. X 40. 



F^G. 74. A more caudal section than Fig. 73. Shows the crista, the 

 cavities, and the small amount of alba. 



PLATE VII. 



Transections of the brain of a larval diemyctylus from which Fig. 63- 

 67 were reconstructed, at the level of the corresponding numbers of 

 Fig. .67. X 65. 



Fig. 75. Through the olfactory nerves. 



Fig. 76. The section cephalad of the portae, shows the extent of the 

 terma, (cf. the cinerea on the mesal view at this level, Fig. 67). 



Fig. 77. Shows the remnants of the double fold of terma, the more 

 ventral of which is the crista. 



Fig. 78. Shows the portae, the plexuses, the cephalic part of the ha- 

 benae, and the tube of the paraphysis. 



Fig. 79. A part of a section between Fig. 77, 78 to show the cephalic 

 enlarged part of the paraphysis and its relation to the membranes. The 

 dura does not extend around it as in the adult. X 150. 



Fig. 80. vShows the dia- and auliplexuses and the sulcus opposite the 

 latter which passes into the infundibulum. 



Fig. 81. Shows the close approximation of the geminums at the dor- 

 sal side and the infundibulum, composed of cinerea. 



Fig. 82. Shows the cephalic prolongations of the lateral recesses of 

 the epicoele and the three parts of the gasserian ganglion. 



Fig. 83. Shows the cephalic parts of the cerebellum which do not 

 unite at this level across the meson. 



Fig. 84. Shows the caudal expansion of the mesoccele, a trace of 

 alba in the cerebellum and a few cells at the origin of the 4th nerve. 



