THE MANDIEULAR AND HYOID MUSCLES OF MAMMALS. 617 
separated into stapedius and hyoid ventral constrictor, extend 
as a continuous strip from beliind the latero-hyale downwards 
and forwards lateral to the stylo-hyale, and then anterior to the 
stylo-hyale to the middle line just behind the posterior edge of 
the intermandibularis. 
In a 17 mm. embryo the anterior digastric is quite separated 
from the intermandibularis. The stapedius portion of the 
hyoid muscle-Anlage is partially separated from the hyoid 
ventral constrictor, and some of its fibres are inserted into the 
back of the upper end of the stylo-hyale. 
In a 19 mm. embryo the crista parotica is formed and the 
upper end of the latero-hyale is continuous with it. The 
stapedius muscle is now quite separated from the hyoid ventral 
constrictor ; it arises from the floor of the fossa stapedii and 
is inserted into the inter-hyale. The hyoid ventral constrictor 
has no dorsal attachment, its upper end lies just below the 
stapedius, and it extends downwards to a ventral aponeurosis. 
The atiterior digastric extends laterally to Meckel’s cartilage, 
whilst posteriorly it extends backwards and upwards on 
the lateral surface of the hyoid ventral constrictor, as in 
the 15 mm. stage. 
In a 21 mm. embryo (figs. 61-63) the inter-hyale has dis- 
appeared and the stapedius is inserted into the stapes. The 
li}"oid ventral constrictor has extended dorsally and its upjier 
end is attached to the paroccipital process, and it has divided 
into dorsal and ventral portions. The dorsal portion is 
partially separated into jugulo-hyoideus and posterior 
digastric. The jugulo-hyoideus is inserted into the stylo-hyal 
cartilage. The lower end of the posterior digastric is con- 
tinuous with the posterior end of the anterior digastric. The 
anterior end of the anterior digastric is attached laterally to 
the Anlage of the mandible. The dorsal end of the ventral 
portion of the hyoid ventral constrictor is attached to the 
stylo-hyale ; from this point it passes downwards to the 
transverse ventral aponeurosis. 
In a 21 mm. embryo the only change is a slightly greater 
separation of the jugulo-hyoideus and posterior digastric. In 
