THE MA^JDTRULAR AND HYOID MUSCLES OF MAMMALS. 573 
On the Development and Morphology of the 
Mandibular and Hyoid Muscles of Mammals. 
By 
F. II. Ffl^^owortli, Itl.D., 
Professor of Medicine, University of Bristol. 
With Plates 38-45 and 9 Text-figures. 
In the course of a paper on tlie morphology of the muscles 
of the head in Vertebrates published in this Journal two 
years ago, I gave a short description of those of the 
mandibular and hyoid segments in the rabbit. There is 
great difldculty in distinguishing rnuscle-Anlagen from sur- 
rounding mesoblast in the early stages of developing Mammals, 
and, not quite satisfied with some of the statements made, I 
have re-investigated the phenomena. Tlie inquiry has been 
much facilitated by Prof. J. P. Hill, who very kindly lent me 
sections of Dasyurus viverrinus — an animal which is 
born with two masticatory muscles only and an incudo- 
meckelian joint, and in which the development of typical 
mammalian muscles and of a squamoso-mandibular joint takes 
place after birth. These changes can be easily followed, and 
enable one to interpret the more obscure phenomena occur- 
ring in Mammals with a longer intra-uterine development. 
Other Mammals have also been investigated, viz. pig, rabbit, 
Phoca vitulina, Halichoerus grypus, Bradypus mar- 
moratus, Dasypus n o vein cin ctu s, Manis penta- 
dactyla, Didelphys aurita. Echidna aculeata, Orni- 
t horhy n chus. 
The first investigator of the development of the masticatory 
muscles was Reuter, who stated that in the pig their Anlage 
is first visible in embryos of 16 mm. Macken-Steisslange in 
