370 DYNAMICS OF HISTOGENESIS. I 



Ameba possesses the property of contractility in all possible directions. 

 The function of contraction in one definite direction characterizes 

 muscle tissue from that of undifferentiated protoplasm. What ini- 

 tiates the progressive series of physicochemical changes in the mesen- 

 chyme resulting in an alteration of its attribute from non-specificity 

 to its specificity of direction of contractility? This question is an- 

 swered as follows. 



The mesenchyme before differentiating into muscle tissue must be 

 subjected to a certain minimal homogeneous and ellipsoidal strain. 

 This strain is objectively evident by an alteration of the form of the 

 spherical nuclei, into the ellipsoidal and spindle conditions and by an 

 elongation of the granular cytoplasm into parallel granular and con- 

 tinuous fibrillae. The fibrillae are arranged along lines of internal and 

 reacting tensional stresses. The ends of the mesenchymal cells, in 

 tension, must be attached to supports of which one, at least, is mobile. 

 The tensional stresses are reactions to simultaneous forces extrinsic 

 to the zone of myogenesis. The external forces are produced by a 

 progressive divergence or separation of the mobile supports to which 

 the mesenchymal cells are attached. Therefore, muscle tissue is not 

 self-differentiating but is dependent upon an external dynamic stimu- 

 lus. As regards smooth muscle this stimulus is the tension of differ- 

 ential growth. 



SUMMARY. 



1. The region of most active mitosis per mm. of cross-section in the 

 intestine is the entodermal epithelial tube. The mitotic figures pri- 

 marily follow the path of a right-handed helix. In one of the twenty 

 embryos the mitotic figures describe the path of a right-handed helix. 



2. The region of least active or relatively passive growth per mm. 

 of cross-section is the mesenchyme, derived from the splanchnic 

 mesoderm surrounding the epitheHal tube. 



3. The rapid expansion, due to epithelial growth in a rotating 

 spiral manner, of the intestinal lumen is greater than the activity man- 

 ifest in the surrounding mesenchyme. This causes a pressure in the 

 latter resulting in a flattening and elongation of the mesenchymal 

 cells. The successive changes in shape of these cells through the 

 spherical, ellipsoidal, and spindle cellular phases are seen. The 



