334 
EDWARD PD ELDS ALLIS. 
inent, or so-called origin, the entire sheet can be turned 
downward a certain distance, disclosing portions of the dorsal 
extrabranchials, the dorsal branchial rays of the hyal arch, 
and the dorsal portions of the five branchial pouches. It is 
then found that four thin sheets of muscle fibres still attach 
the large muscle-sheet to underlying structures, and if these 
thin sheets be cut the large muscle-sheet can be turned 
downward to the middle line of the gill-openiugs, as shown 
in PL 22, fig. 9. The entire distal portion of each doi>al 
extrabranchial is then exposed, and it is seen that they each 
lie against the anterior (external) surface of the gill-pouch 
next posterior to the arch to which the extrabranchial 
belongs, that this distal portion of each of the four branchial 
dorsal extrabrauchials lies slightly posterior to and parallel 
to the dorsal edge of the gill-poucli next anterior to its arch, 
and that it extends ventro-posteriorly slightly beyond the 
level of the dorsal edge of the external opening of the latter 
pouch. The dorsal extrabranchial of the hyal arch is short, 
and extends but a short distance along the anterior (external) 
surface of the first gill-pouch. 
The four linear aponeuroses of the large muscle-sheet are 
now seen to extend entirely through the sheet and to lie one 
external to each of the four branchial dorsal extrabranchials, 
attached to them only by loose connective tissues, and com- 
parison with Dohrn’s and Edgeworth’s descriptions of the 
differentiation of the musculi adductores in embryos of these 
fishes definitely shows what the aponeuroses are. According 
to both those authors those primarily continuous muscle fibres 
(strauds) of the constrictores that cross the inner branchial 
bars of their respective arches acquire insertion on those bars 
along the lines where they cross them, and, as a result of this, 
the musculi adductores are cut out of the primarily long and 
continuous fibres (strands) so concerned. But before that 
insertion of these fibres (strands) was acquired it is evident 
that the individual fibres concerned must first have become 
tendinous along the lines where they were later to be cut in 
two, this doubtless being due to the interruption of the 
