neal: nekvous system in squalus acanthias. 235 



3. Trochlbaris. 



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 

 ixnd, deni Anschein wenigsteus nach, vollstandig structurlose Fiiden, 

 welche in ihrer ganzen Ausdehnung vom Gehirndach bis zum ent- 

 sprechenden Muskel verfolgt werden konnen." The great variety of 

 opinions concerning the morphology of this eye-muscle nerve make in- 

 teresting the facts of its development. Hoftmann ('89, p. 338), who 

 was the first to study its development, states that in Lacerta one iinds, 

 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 ziemHch grosses Ganglion, 

 welches aber friihzeitig wieder vollstandig abortirt." ^ In later stages 

 of development the trochlear emerges as " dtinner, feinfaseriger Nerven- 

 stamm von der oben erwahnten Stelle aus dem Gehirn und wird in 

 seinem weiteren Verlauf bald so schmachtig, dass er nur aus einzelnen, 

 sehr dlinnen 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 Auslauferbildung der Ganglienzellen." According 

 to Miss Piatt ('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 



^ Confirmed by Oppel, '90. 



2 Miss Piatt makes, in my opinion, an unnecessary distinction between a " pri- 

 mary " and a secondary, or " permanent " trochlearis. The " primary trochlearis " 



