EBEN J. CAREY 371 



mesenchymal wall decreases in thickness, due to tension caused by 

 epithelial tubular dilation. 



4. The rotating spiral growth of the epithelial cells causes the forma- 

 tion of a series of mesenchymal cellular and fibrillar concentric rings 

 due to the centripetal force of the former. 



5. The circular, smooth muscle cells are differentiated in the outer, 

 more condensed margins of the ring. At these points the developing 

 tensional stresses are greater than within the ring. 



6. The inner circular smooth muscle coat is the first one differenti- 

 ated and is incident to the rapid growth of the epithelial tube in di- 

 ameter. The former soon tends to restrict the growth of the epi- 

 thelial tube in diameter. The tube, pursuing the lines of least re- 

 sistance, grows in length. During the period of rapid growth in length 

 the outer longitudinal muscle coat is in the process of formation. 



7. The tensional stresses to which the elongated strained mesen- 

 chymal cells are subjected appear to be a dynamic stimulus to smooth 

 muscle differentiation. 



8. From this study of a closely graded and progressive series of sec- 

 tions of intestinal development, the conclusion is drawn that muscle 

 tissue is not self-differentiating, in the strict sense of the term, but 

 that the tension of differential growth acts as the stimulus to smooth 

 muscle differentiation. 



The writer wishes to express his indebtedness to Professor H. von 

 W. Schulte for his interest and his valuable suggestions; to Madame 

 Helen Ziska for the illustrations; and to his wife for her help in reading 

 the proof. 



BIBLIOGRAPHY. 



Bardeen, C. R., Johns Hopkins Hosp. Rep., 1900, ix, 231; Am. J. Anat., 1906-07, 



vi, 259. 

 Boveri, T., Sitzungsh. phys.-med. Ges. zu Wurzburg, 1904, 16; Ergebnisse iiber 



die Konstitution der chromatischen Substanz des Zelikerns, Jena, 1904, 



115. 

 Carey, E. J., Anat. Rec, 1917, xi, 1; 1918, xiv, 30; 1919, xvi, 45, 114. 

 Child, C. M., Biological lectures from the Marine Biological Laboratory of Woods 



Hole, 1899, 231; Individuality in organisms, Chicago, 1915, 5. 

 Conklin, E. G., Biological lectures from the Marine Biological Laboratory of 



Woods Hole, 1896-97, 17; /. Exp. ZooL, 1905, ii, 145. 



