208 University of California Publications in Zoology. [ VoL - 4 



ventral extension, so that the mesoderm forms a broken ring 

 around the anterior extremity of the enteron (fig. 27). From 

 this point the mesodermal growth is confined to the sides. (See 

 quotation from Bateson.) 



The following description refers exclusively to the lateral 

 backward growth of the mesoderm. In the section represented 

 in fig. 24, pi. 8, it will be seen that the walls of the enteron are 

 completely separated from the anterior body cavity except at one 

 point. Complete separation is seen in stages represented by 

 figs. 27 and 28. The walls of the anterior body cavity may now 

 be called mesoderm. 2 The edges of the anterior portion of the 

 enteric walls are approaching, leaving a small oblong communi- 

 cation (oblong dorso-ventrally) between the two cavities. 



The growth of the mesoderm backward is taking place wedge- 

 wise between the walls of the enteron and the ectoderm. The 

 cells of the edge are irregular, tending somewhat to flatness at 

 its narrow extremity. In this way the mesoderm continues to 

 grow backward, the wedge of cells (as seen in longitudinal sec- 

 tion) becoming somewhat longer and thinner, and reaching be- 

 yond the collar region. This stage of development is shown in 

 fig. 25. At a little later period an important change takes place. 

 The mass of cells extending backward between the enteric wall 

 and the ectoderm loses its wedge-shape in longitudinal section. 

 At a point corresponding to the anterior collar groove, the mass 

 of cells is slightly constricted. In the collar region corresponding 

 in longitudinal extent to the collar, the cells form two layers. 

 Posterior to this region the mesoderm continues for a short dis- 

 tance as a single layer of somewhat flattened cells. (Fig. 26.) 

 This is the beginning of the middle body cavity. 



The mesoderm grows backward until it almost reaches the 

 posterior extremity of the enteron. In the meantime a second 

 constriction occurs at a point corresponding to the posterior 

 groove of the collar (fig. 28, pi. 8). Posterior to this constriction 

 tin- mesoderm forms two layers just as described above for the 

 collar region. 



In this manner the middle and posterior body cavities arise 



The term mesoderm has already been used in reference to the portion of 

 this region which is reflected backward. 



