On the Development and Morphology of the Mandibular and Hyoid Muscles of Mammals

IN the course of a paper on the 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 mandibnlar and hyoid segments in the rabbit. There is great difficulty in distinguishing muscle-Anlagen from surrounding mesoblast in the early stages of developing Mammals, and, not quite satisfied with some of the statements made, I have re-investigated the phenomena. The 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 iucudomeckelian 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 occurring in Mammals with a longer intra-uterine development. Other Mammals have also been investigated, viz. pig, rabbit, Phoca vitulina, Halichoerus grypus, Bradypus marmoratus, Dasypus no vemcinctus, Manis pentadactyla, Didelphys aurita, Echidna aculeata, Ornithorhynchus.