98 PL ATT. [Vol. V. 



micus profundus trigemini. Along the same line the rudi- 

 mentary ganglion is continued forwards to unite with cells, 

 which, arising from the anterior lips of the neuropore, have 

 fused with the primary nasal epithelium. Successive stages in 

 the early development of this nerve have not been figured, from 

 lack of abundant material. Recently, however, the needed 

 material has been obtained, and I hope soon to give the devel- 

 opment and relations of the nerve more careful study than has 

 hitherto been possible. 



The nerve arises from the constriction that separates mesen- 

 cephalon and thalamencephalon. As the term " mesocephalic " 

 has been applied by Beard (No. 3) to the ganglion usually called 

 "ciliary," I suggest that this new nerve, in consideration of its 

 origin, be called the " thalamic " nerve. With the growth of 

 the dorsal wall of the brain, the nerve is carried forwards and 

 comes to lie directly above the optic stalk. In Fig. 1, Th, PL IV, 

 the position of the nerve is outlined at the time when one gill 

 cleft has broken through. Sec. 12, PL VI, passes through the 

 root of the thalamic nerve at the time when two pairs of bran- 

 chial clefts have broken through, and the third is only closed by 

 the thinnest ectoderm. The plane of this section is indicated 

 in Fig. 1, sec. 11, PL IV : the section, however, is from an older 

 embryo. The nerve at this stage is a round fibrous cord con- 

 taining many nuclei. Its fibres can be traced into the brain, 

 but vanish distally in the mesoderm above the eye. The prim- 

 itive character of undifferentiated cells from the neural crest 

 has been entirely lost. Later the thalamic nerve atrophies in 

 much the same manner as does the primary trochlear. 



In the series from which sec. 3 is drawn, the remains of 

 the thalamic nerve are still to be found as two masses of cells, 

 one connected with the brain in the mid-dorsal line, the other 

 lying isolated at the side of the brain. This tells us that the 

 thalamic nerve does not entirely disappear before the oculomo- 

 torius is established, or before four pairs of branchial clefts have 

 broken through. The phylogenetic significance of these facts 

 is surely great; and although the homologue of this nerve, found 

 in other Selachians is a quickly vanishing prolongation of the 

 neural crest, it is not therefore to be lightly overlooked. 



When Rabl (No. 15) says, "Vor allem haben wir daran festzu- 

 halten, dass der Vorderkopf nur zwei primare Nerven, den Tri- 



