THE .AfANDIBULAH ANJ) IIYOID MUSCLES OE ,AIAMHALS. G15 
In a 11 mm. embryo tlie Anlage lias split into a superficial and 
a deep layer — the former is the Anlage of the platysma,, the 
latter that of the stapedius, digastricns, and stylo-hyoid. In 
a 15 mm. embryo the antero-posterior fibres of the stapedius 
form the most dorsal portion of the Anlage ; immediately 
below these is the upper end of the digastricns, i. e. this 
muscle has not yet extended up to the mastoid process. The 
digastricus is in two parts, of which the posterior (innervated 
by the N. facialis) is inserted into Keichert^s cartilage ; the 
anterior passes round the cartilage and is inserted in Meckehs 
cartilage. A small outgrowth from the posterior part is the 
Anlage of the stylo-hyoid. In a 21 mm. embryo the upper 
end of the digastricus has spread up to the mastoid process, 
and the jngulo-hyoideus has separated from its upper part and 
is inserted into the hyoid bar. The stylo-hyoid is fully 
separated. 
Kallius (1909) described the development of some of the 
hyoid muscles in his account of the tongue and associated 
structures of the pig. In stage 12 (= KeibeTs stage 22) 
‘Won dem proximalen Eude des Knorpelanlage des zweiten 
Schundbogen zieht ein schmaler Strang von wenig differen- 
zierten Muskelblastem nach der Mittellinie zu in die Gegend 
des oralen Endes der Copula des Branchialskeletes.^^ There 
is no trace of an anterior digastric. In stage 17 (= between 
twenty-seventh and twenty-eighth stages of Keibel) the 
anterior digastric forms a well “ abgrenzbaren Muskelbundel 
welcher offeubar mit der ersten Portion (Mylohyoideus) von 
der urspriinglich einheitlichen Trigeminus muskulatur abgez- 
weigt hat ; its inner end is continuous by a tendon with the 
posterior digastric, the lower part of wliichis almost surrounded 
by the stylo-hyoid. A little higher up these two muscles 
diverge, the stylo-hyoid being affixed to the h}^oid cartilage, 
the posterior digastric to the base of the skull. Both posterior 
digastric and stylo-hyoid receive branches from the X. facialis. 
(The earliest stages of development of these muscles were not 
described.) In stage 27 (embryo 92 mm. in greatest length) 
a jugulo-hyoideus is formed probably from a small part of the 
VOL. 59, PART 4. NEAV SERIES. 40 
