EBEN J. CAREY 71 



elongation. Those at the periphery are more stretched than those 

 centrally located. The nuclei are first elongated in the direction of 

 the epithelial growth and shortly thereafter granules are found in 

 the intercellular and intracellular cytoplasm. These granules are' 

 arranged in rows, likewise in the direction of the growth of the epi- 

 thelial tube. Yet the distribution of the cells is fairly uniform through- 

 out. No cell boundaries are presented. The entire region is a syncy- 

 tium studded with nuclei surrounded by a narrow zone of granular 

 endoplasm. The ectoplasm forms the connecting phase which char- 

 acterizes the unity of organization found in the mesenchyme. 



The epithelial tube, larger in the 11.5 mm. pig embryo than in the 

 9.5 mm. pig embryo (Fig. 1), possesses an increased number of cells. 

 Numerous mitotic figures may be plotted in serial sections (Fig. 5). 

 These form the path of a definite left-handed helix from above down- 

 ward (Figs. 7 to 14). 



The left-handed helix of the large intestine is also followed by 

 the epithelial mitosis but from below upwards. The ileocecal valve 

 is one point of convergence of the helicoidal growth of the epithelial 

 tube. The increased growth in diameter of the large intestine with 

 the path of mitosis directed cephalad meets the smaller diameter 

 of the ileum with its mitotic path directed caudad. These are me- 

 chanical factors involved in the formation of the definite valve-like 

 constrictors in the ahmentary tract. 



As development continues to the stage of a 14 mm. embryo there 

 is a rapid increase in size of the epithelial tube. The lumen, although 

 larger, is not correspondingly as large as the entire tube. The in- 

 crease in size of the latter, therefore, is due to an increase in the num- 

 ber of cells crowding the lumen. 



Concomitant with the rapid growth of the epithelial tube a begin- 

 ning of formation of the apparent ring of stretched cells is found at the 

 periphery of the mesenchymal vortex arranged in a tangential posi- 

 tion. Various degrees in the elongation of the mesenchymal cells 

 are found decreasing in intensity of stretching from without inwards. 



On the outer side of the myoblastic ring the intercellular substance 

 is drawn out into myofibrillae. On the inner side of the ring, discrete 

 granules are arranged in the same tangential direction as the elon- 

 gated nuclei of the myoblasts. That the same force produces these 



