438 BASHFORD DEAN. 
also been reviewed in the table and figuresof pp. 483-434. By 
the time of the blastopore’s closure it has given rise to the 
sagittal thickening out of which the central nervous system is 
shortly to be formed. This, in its early characters, suggests 
closely the neuron of the Teleost (Pl. 82, figs. 34 and [trans- 
verse section] 86). Its deep keel-like thickening resembles 
closely as well the earliest conditions of Lepidosteus, while 
diverging from the type of Acipenser. To the conditions in 
this form, however, an apparent similarity exists in the stage 
already noted in Pl. 30, fig. 16, where a slight axial groove is 
for a short time present; this condition, of interest, accord- 
ingly, from its sturgeon-like feature, is further illustrated in 
the transverse section of Pl. 82, fig. 87; the axial groove is 
never deeper than here figured, and shortly passes away, 
flattening as the neuron increases in size and depth. The 
lumen which the neuron later acquires takes its definite origin 
in the dissociation of cells in the vertical plane of its axis 
(Pl. 32, fig.39). This condition is clearly to be compared with 
that of the Teleost, as described by Hoffman, v. Kupffer, H. 
V. Wilson, and others. It might, moreover, be regarded as 
confirming conclusively the position of v. Kupffer in regard to 
the morphological importance of the neural furrow of the 
Teleost, i.e. that it is homologous to the primitive neural 
furrow of Amniotes; and, on the other hand, it certainly 
removes the ground for believing that the neural axis of the 
Teleost was primitively solid, as Minot has maintained.'! For 
there can be little doubt that Elasmobranchian characters are 
present in the origin of the neuron of Acipenser, and it follows, 
therefore, that the neural furrow of Amia, its ally, must 
represent, if only in a transient way, the early Amniote con- 
dition. In the Teleost, accordingly, it is reasonable to expect 
that abbreviated growth stages have greatly reduced the 
prominence of the ancestral medullary folds, while perfecting 
the newly acquired mode of securing a neural canal. 
A further and striking similarity to Teleostean conditions is 
found in Amia in the early development of the optic vesicles. 
1 « American Naturalist,’ November, 1889. 
