586 Proceedings of the Royal Society of Edinburgh. [Sess. 
third of the first pharyngeal cleft, the mesenchymic area known as the 
first visceral arch thickens medio-laterally. Moreover, it begins to extend 
slightly cranially and strongly caudally, enclosing the chorda tympani, 
until it almost merges with . the hyoid arch. Now the more cranial portion 
extends in two directions, one dorso-laterally, the other dorso-medially. 
We have here the first anlage of the incus, with its crus longum and crus 
breve. The long cranio-caudal area of mesenchyme is the anlage of the 
malleus. At this time the mesenchymic mass encloses both structures , 
although their later forms are to be distinguished by certain cell differentia- 
tions. This distinction is important, and it is on this basis that I have 
modelled this embryo. The model does not show cartilage and pro-cartilage 
as distinct from the surrounding mesenchyme — but rather the whole mesen- 
chymal extent has been modelled. 
Kastchenko, together with some other investigators, has declared that 
the external auditory meatus has nothing to do with the spiracular or first 
visceral cleft. But inasmuch as the present discussion depends little upon 
whether the external auditory meatus be the first gill furrow or not, it is 
of no good to enter into that discussion here, except merely to state that 
the view of Kolliker, Gaupp, and others, that the external auditory meatus 
is derived from the first gill furrow, seems to have vastly more weight than 
the contrary hypothesis. 
The first pharyngeal pouch of the five- weeks embryo will be seen to 
arise somewhat caudal from the first gill furrow and to extend cranio- 
laterally for two-thirds of its length. Then it bends sharply, and for the 
remainder of its length extends medio-cranially. This medio-cranial 
flexure has apparently escaped Fuchs’ notice. It is marked in the diagram 
with an asterisk. Now it is evident that in the ideal condition, to speak of 
a structure as cranial to the first pharyngeal pouch would throw it at once 
into the territory of the mandibular arch. But as Fuchs rightly points out, 
to say that a structure is cranial to the first pharyngeal pouch when the 
relations are those of this stage, does not of necessity make it lie in man- 
dibular territory, for the mandibular arch in this case lies lateral to the 
first pharyngeal pouch. Moreover, the first pharyngeal pouch is inclined 
somewhat dorsally, as is shown in fig. 1, and consequently it is conceivable 
that a structure ( i.e . stapes anlage in diagrams) might have become so dis- 
placed by pressure and unequal growth as to lie medial and dorsal to the 
first pharyngeal pouch and still retain its ontogenetic relation with the first 
visceral arch. 
Reference to text fig. 1, B, and to figs. 1 to 3, will show that the 
mesenchyme has grown between and separated the first pharyngeal pouch 
