242 Caroline McGill, 



as in the circular layer. The circular muscle is seen in cross section, 

 with finer myofibrillae appearing, fc = coarse myofibrillae in a cell of 

 the circular layer; //"= finer fibrillae; vf ^ varicose myofibrilla in the 

 longitudinal layer; mm = muscularis mucosae; e = endoderm becoming 

 stratified; gm = ganglion of Meissner's plexus; {/a =^ ganglion of Auer- 

 bach's plexus. Zenker's fluid. Iron-haematoxylin and eosin. x 1045. 



Plate IX. 



Fig. 9. Cross section of a portion of the lower oesophagus of a 15 mm embryo. 

 All of the myofibrillae {fc), the developing collagenous fibrillae {cf) 

 and most of the granular protoplasm are stained blue. The nuclei and 

 some scattered protoplasmic granules are stained reddish-yellow. The 

 collagenous fibers are forming throughout the entire mesenchymal syn- 

 cytium, including the muscle. Zenker's fluid. Mallory's anilin-blue con- 

 nective tissue stain, x 1425. 



Fig. 10. Cross section of a portion of the lower oesophagus of a 30 mm embryo, 

 showing a marked increase in the number and size of the collagenous 

 fibrillae. The myofibrillae (fc) are differentially stained a bluish or 

 reddish-yellow. The relation of the collagenous fibrillae {cf) to the 

 muscle syncytium is well shown. Zenker's fluid. Mallory's anilin-blue 

 connective tissue stain, x 1425. 



Fig. 11. Cross section of the lower oesophagus of a 38 mm embryo, showing a 

 portion of both muscle layers. The fine myofibrillae are now abundant, 

 especially in the circular layer. In the longitudinal layer, cut ends of 

 both coarse and fine myofibrillae are visible, though the fine myofibrillae 

 are here indistinguishable from protoplasmic granules, òr = shows a 

 coarse myofibrilla breaking up into a brush of fine fibrillae; ct = an 

 interstitial embryonal connective tissue cell. Zenker's fluid. Delafield's 

 haematoxylin and eosin. x 1425. 



Fig. 12. Ci'oss section of a portion of the lower oesophagus of a 76 mm embryo, 

 through the subepithelial tissue and the circular muscle layer. The 

 muscularis mucosae shows as scattered muscle cells in the subepithelial 

 embryonal connective tissue. Since this layer is made up of longitudinal 

 muscle fibers, they are here shown cut in cross section. Further diffe- 

 rentiation of smooth muscle for the increase in thickness of the circular 

 layer is shown at x, where embryonal connective tissue cells are elongating, 

 and at y, where one of the interstitial embryonal connective tissue cells 

 is elongating. In both these areas of new formation, all of the myo- 

 fibrillae arising are fine, homogeneous fibrillae. mi =^ increase in muscle 

 elements by mitosis; cp = blood capillary; mm = muscularis mucosae; 

 mu = muscle cell of the musculai'is mucosae, showing both fine (//) 

 and coarse [fc) myofibrillae; b = basement membrane; bi' = a coarse 

 myofibrilla breaking up into a brush of fine homogeneous fibrillae, 

 Zenker's fluid. Delafield's haematoxylin and eosin. x 1425. 



Fig. 13. A portion of a cross section of the lower oesophagus of a 76 mm embryo, 

 showing a part of the circular muscle layer. Here the myofibrillae and 

 granular protoplasm show the differential yellowish-red stain, the collage- 

 nous .fibrillae appear blue, cf = collagenous fibrillae, mostly fine, but 



