220 Caroline McGül, 



chyme is true, however, only for the beginning of the circular layer 

 in the earlier embryo. As will be described later, in embryos between 

 15 and 20 mm in length, throughout the mesenchyme of the entire 

 digestive tract, the formation of collagenous fibrillae begins, so that 

 the primitive mesenchyme is thus transformed into what may now be 

 called embryonal connective tissue. From this embryonal connective 

 tissue syncytium, the remainder of the circular layer of smooth muscle, 

 the entire longitudinal layer, and the muscularis mucosae are developed. 

 In the areas of muscle formation, not all of the mesenchymal 

 cells (or, in later development, of the embryonal connective tissue 

 cells) elongate. Some of them retain their stellate shape with oval 

 nuclei {d, in Figs. 7 and 11). From these cells, in later develop- 

 ment, muscle cells may arise; but they form, in the main, the anläge 

 of the interstitial connective tissue. This will be discussed later under 

 the development of the connective tissue. 



2. Formation of Muscle Layers in the Alimentary Canal. 



The elongation of the mesenchymal nuclei to form the muscle 

 of the circular layer occurs first, as already stated, in the oesophagus 

 at the level of the bifurcation of the trachea, and makes its appe- 

 arance in the 5 mm pig embryo. From this point the elongation 

 proceeds up and down the digestive tract. It has reached the upper 

 and the lower oesophagus in an 8 mm embryo (Fig. 2). It does not 

 appear in the stomach until in the 9 mm pig, and has extended 

 throughout the entire alimentary canal by the time the embryo has 

 reached a length of 20 mm. 



The longitudinal muscle follows the same course as does the 

 circular layer, appearing first in the mid-oesophagus, and from there 

 spreading both downwards and upwards. It appears much later than 

 does the circular layer, the elongation of nuclei for its formation first 

 showing in embryos from 20 mm to 25 mm in length. Somewhat 

 later, in about the 30 mm pig, it makes its appearance in the stomach 

 and from there gradually forms throughout the length of the intestine. 



The muscularis mucosae develops somewhat later than the other 

 layers. In the oesophagus of the 27 mm pig there is a slight elon- 



