20 F, L. LANDACRE 
the ventral portion takes part in the formation of the mesen- 
chyme and cartilage of the gill bar. 
In the last stage plotted (fig. 11) from stage 1 of series IJ, 
6 mm. long, and approximately twenty-four hours older than 
that from which figure 10 was taken, the ectodermal mesenchyme 
has reached, except in the posterior pharyngeal region, its final 
distribution. Its later history involves its differentiation as 
distinct from its migration. 
In the anterior head region, including the mandibular bar, 
the changes in distribution are both progressive and regressive. 
The most marked increase in extent of ectodermal mesenchyme 
in this region is in the mandibular bar due apparently to the 
growth of the bar. There isalsoaslight increase in the extreme 
anterior end of the head or rather a residue in the form of spur 
of ectodermal mesenchyme extending dorsally from the position 
of the epiphysis.. This spur lies morphologically on the dorsal 
wall of the midbrain. In the region of the olfactory capsule the 
ectodermal mesenchyme forms a continuous sheet between the 
forebrain and ectoderm. It also forms a continuous sheet 
between the anterior border of the optic vesicle and ectoderm 
with a second spur extending dorsally to the level of the pro- 
fundus ganglion and a third extending ventrally under the olfac- 
tory and optic capsules which is continuous with the ectodermal 
mesenchyme of the mandibular bar. Except for the first two 
spurs mentioned, the ectodermal mesenchyme is now confined 
as a continuous sheet to the anterior and ventral head regions 
anterior to the level of the gasserian ganglion. This represents 
a rather marked decrease in the relative extent of the ectodermal 
mesenchyme in the dorsal portion of the anterior head region. 
In the posterior head region there is in this stage a marked 
ventral extension of the crest at the levels of V, VII, IX, and X 
ganglia. The ectodermal mesenchyme at these levels has grown 
into the corresponding gill bars and now reaches to the level of 
the heart and ventral aorta, covering from the lateral view the 
whole of the entodermal derivatives of these bars. Except in 
the posterior bar, the ectodermal mesenchyme has further 
completely surrounded the endodermal derivatives. Posterior 
