324 
HENEY OER. 
physis, and the mouth-fusion, with their relative changes of position. 
(Z. 2, A A.) 
Fig. 24 g. — Part of a longitudinal median vertical section of an embryo of 
Amblystoma, showing the floor of the hind-brain and the fold between the 
hind-brain and infundibulum, which is caused by the primary cranial flexure. 
(Z. 2, A.) 
Fig. 25. — Taken from a series of longitudinal vertical sections of an embryo 
of Amblystoma ; it shows the balancer in nearly longitudinal section, and also 
a superficial portion of the mandibular arch. (Z. 2, A.) 
PLATE XXIX. 
Fig. 26. — Horizontal section of an embryo of Amblystoma at a stage 
between the stages e — F and the stage G. This figure shows the first formation 
of the optic lens, also the formation of the gill-clefts and the somites of the 
head. (Z. 2, A A.) 
Figs. 27 n — 30 h inch — Horizontal sections of an embryo of Amblystoma 
at the same stage as the embryo of Fig. 18. Of these sections, 27 H is the 
most dorsal and 30 h the most ventral, the others being intermediate in the 
order of their numbers. The sections show the development of the nerve- 
fibres in the brain, and the early development of the cartilaginous skeleton 
and the branchial apparatus. (Z. 2, A A.) 
Fig. 31. — Approximate reconstruction of an early stage of the cranial and 
visceral skeleton of Amblystoma, made from drawings of series of sections 
cut in three planes at right angles to each other. It shows the skeleton of 
the head viewed from the left side, also the shape and relative position of the 
left rudiment of the pectoral girdle (P. g.). 
Fig. 32 g. — Lateral longitudinal vertical section of the brain of an embryo 
of Amblystoma, cut through the left side, showing the course of the nerve- 
fibres at the time of their first development in the brain. (Z. 2, A.) 
Fig. 33. — Part of a transverse section through the fore-brain of a Frog- 
embryo, showing the fibres of the lateral and anterior band ( L . F. and A. F.), 
the latter crossing the anterior surface of the brain ; also the fibres developing 
on the anterior wall of the optic stalk ( n . II). The posterior wall of the 
optic stalk (o. y;".) is free from fibres. (Z. 4, A.) 
Fig. 34. — Transverse section of the spinal cord in the dorsal region of an 
embryo of Amblystoma at the stage of series n. It shows in cross-section 
the longitudinal nerve-fibres of the lateral band (L. P.), also the transverse 
fibres of the ventral commissure ( T . F.). (Z. 4, A.) 
Fig. 35. — Transverse section passing through the mid brain and secondary 
fore-brain of an embryo of Amblystoma at the same stage as the embryo of 
series g. It shows the corpora striata (si.) and the transverse fold which 
separates the secondary fore-brain from the thalamencephalon. P. C. shows 
the position of the posterior commissure. (Z. 2, A A.) 
