NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 235 
3. TROCHLEARIS. 
In an embryo about 21-22 mm. in length (Plate 4, Fig. 21) the 
trochlearis, the last cranial nerve differentiated, appears, as stated by 
Kastschenko (’88, p. 465), in the form of “parallel gehende kernlose 
und, dem Anschein wenigstens nach, vollständig structurlose Fäden, 
welche in ihrer ganzen Ausdehnung vom Gehirndach bis zum ent- 
sprechenden Muskel verfolgt werden können.” The great variety of 
opinions concerning the morphology of this eye-muscle nerve make in- 
teresting the facts of its development. Hoffmann (’89, p. 338), who 
was the first to study its development, states that in Lacerta one finds, 
as the Anlage of the trochlearis, “einen ziemlich grossen, zelligen Aus- 
wuchs” between midbrain and hindbrain. At certain stages the trochle- 
aris possesses “ein sehr deutliches und zwar ziemlich grosses Ganglion, 
welches aber frühzeitig wieder vollständig abortirt.”1 In later stages 
of development the trochlear emerges as “ dünner, feinfaseriger Nerven- 
stamm von der oben erwähnten Stelle aus dem Gehirn und wird in 
seinem weiteren Verlauf bald so schmächtig, dass er nur aus einzelnen, 
sehr dünnen Fasern besteht.” In other reptiles, in birds, and in car- 
tilaginous fishes, Hoffmann was unable to find evidence of this gan- 
glion of the trochlearis. In 1890 and 1891 Dohrn announced that, in 
early stages of the development of the trochlearis, erratic ganglia, which 
were evidently products of the neural crest, are found in Selachian 
embryos in connection with this nerve. Whether these ganglia send 
fibres into the trochlearis stem, he was not able to determine. In later 
stages anastomosing fibres appear to connect the trochlearis with the 
r. ophth. sup. V and VII. Moreover, Froriep (91) thinks he is able to 
establish in Torpedo the genetic connection of a pear-shaped ganglion 
with the trochlearis. From his studies upon Torpedo embryos, he is 
also forced to conclude that the trochlearis arises in situ through the 
“ Umwandlung oder Ausläuferbildung ‚der Ganglienzellen.” According 
to Miss Platt (91, p. 95), the trochlearis in Acanthias first appears as a 
small fibrous nerve growing from the constriction between midbrain and 
hindbrain, This may be followed a short distance into the mesoderm, 
but, becoming extremely attenuated, is soon lost. “Soon after the 
appearance of this small nerve, which is the root of the permanent 
trochlearis,? cells are proliferated to meet it from the ganglion cells that 
1 Confirmed by Oppel, ’90. 
2 Miss Platt makes, in my opinion, an unnecessary distinction between a “ pri- 
mary” and a secondary, or “permanent” trochlearis. The “ primary trochlearis ” 
