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Neural Segments in Salmo. Living Specimens. 
The fact that neural segments may be observed in the living 
embryo is sufficient evidence that these joints are normal structures 
and not artefacts produced by the reagents necessary to harden and 
preserve tissues. The embryos of the Trout are very transparent 
during the early stages, therefore the segmental grooves on the inner 
surface of the brain wall can be as readily observed as those on the 
outer surface. To do so satisfactorily orientation and manipulation 
of material becomes an important factor. It is very necessary that 
the embryo should be studied from all points of view and on this ac- 
count a good dissecting microscope is usually preferable to the com- 
pound instrument. 
Figs. 1, 2, 3 and 4 are camera drawings made from living spe- 
cimens. The youngest living embryo in which I have found these 
segments is represented by Fig. 1. 
In this specimen, 11 transverse dorsal grooves could be counted 
which divide the encephalon into 11 nearly equal segments or joints 
(1—11). Each groove, therefore, forms a dividing line between ad- 
jacent segments. One of these (f) is deeper than the other grooves 
and lies mid-way between the optic and the auditory vesicles. This 
one marks the posterior border of segment 6, which later develops 
into the cerebellum. This groove becomes very conspicuous in later 
stages and forms conveniently an anatomical land-mark by means of 
which the dividing line between cerebellum and medulla can be iden- 
tified. 
Fig. 1. Salmo purpuratus. Anterior portion of a living embryo with 19 
somites, 16 days old. Left profile view. Z, 2, 3 ete. neural segments; c the anterior 
limit of the mid-brain; f the anterior limit of the medulla; op. ves. optic vesicles. X 60 
diameters, 
