222 Caroline McGill, 



the alimentary canal varies at different times. Especially is this true 

 in the oesophagus, and is most noticeable in the circular layer. From 

 the first appearance of this layer (Figs. 3 and 4) in embryos of from 

 5 mm to 10 mm in length, up to the 15 mm embryo (Fig. 5) the 

 muscle increases very rapidly in amount. This increase is almost ^ 

 entirely by additions from the mesenchyme. Mitosis is abundant in 

 the mesenchyme (Fig. 5, mt), but at this stage very seldom occurs 

 in the elongated nuclei of the already formed muscle cells. In the 

 oesophagus, from the 15 mm up to about the 45 mm embryo, although 

 the muscle tissue already formed is further differentiating by the 

 rapid formation of myofibrillae (cf. Figs. 5 and 11) and increase in 

 the size of the already elongated nuclei, there is little formation of 

 new muscle in the circular layer, so little that the layer already 

 formed may even show a diminution in thickness. This diminution is 

 due to the stretching caused by the very rapid increase in the dia- 

 meter of the oesophagus. At this time, however, the longitudinal 

 layer of muscle is forming. 



From the 45 mm embryo on, there is in the circular layer of the 

 oesophagus a second period of muscle growth which continues until 

 the adult form is reached. This increase, at least up to the fuU term 

 pig, is apparently due to two factors, — first, by the differentiation 

 of the embryonal connective tissue cells, both at the margins of the 

 already formed muscle layer (Fig. 12, ic), and also apparently of those 

 lying between the muscle elements (Fig. 12, y)\ second, by mitotic 

 division of the already formed muscle nuclei (Plate IX, Figs. 12 and 

 14, mt). The former process seems to predominate in the earlier 

 embryonic stages, the latter in later stages. 



The longitudinal layer and the muscularis mucosae of the oeso- 

 phagus, and all of the muscle layers of the stomach and the intestine, 

 show a more continuous growth, without the great variability noted 

 in the circular layer of the oesophagus. 



4. The Formation of Myofibrillae. 

 Immediately following the process of elongation of the mesenchyme, 

 or later of the embryonal connective tissue, the myofibrillae are formed 



