472 BULLETIN 82, UNITED STATES NATIONAL, MUSEUM. 



In this way the vertical mesentery has been formed in the compass of the 

 right ccelome, and the ventral mesentery lies a little to the right of the median 

 plane, between radii IV and V. Toward the foremost end it approaches the 

 median plane more closely and passes over directly into the wall of the furrow 

 described above. 



The vertical mesentery posteriorly does not run dorso-ventrally, but obliquely, 

 more or less parallel to a line joining the outer ends of radii IV and V. There 

 is, however, much individual variation regarding its direction as well as its length. 

 Anteriorly it becomes longer and more nearly parallel to the median plane, since 

 here the right extremity of the left no longer covers it. 



During these changes the place of origin of the chambered organ remains 

 at the anterior end, moving only slightly anteriorly. The individual chambers 

 are usually very easy to recognize; through the thinning of their walls their 

 lumina have become more extensive. 



At this stage the so-called axial organ first makes its appearance. At about 

 the twenty-eighth hour there is to be seen a simple thickening of the left wall 

 of the vertical mesentery directly behind the place on which the chambered organ 

 appears. This thickening consists in an increase in height of the epithelial cells. 

 Nuclear divisions parallel to the surface of the epithelium can be observed which 

 later bring about the several-layered condition of this region. A small cell group 

 pushes itself out and advances between the bases of the two ventral tubes of the 

 chambered organ. This is the most anterior end of the axial organ. 



In somewhat later stages the axial organ without exception lies near the 

 vertical mesentery in the left extremity of the right ccelome. It shows itself 

 first as a thickening of the wall of the mesentery jutting out into the ccelome. 

 The nuclei of its cells resemble those in the cells of the remaining portions of the 

 ccelome wall. 



Somewhat later it has doubled in length, and has come to lie entirely on the 

 mesentery. Anteriorly, toward the column, it has only united with the left wall 

 of the mesentery for a short distance, but posteriorly the union is more extensive. 



The. nuclei of the constituent cells are now not all alike, for some have become 

 larger and vesicular, and are surrounded by a thicker plasma zone. 



Posteriorly the thickening in the left wall of the mesentery gradually flattens 

 itself out before it reaches the hinder border of the ccelome wall. 



Anteriorly where the axial organ lies entirely separate between the two walls 

 of the vertical mesentery it can be followed as far as the calyx border. Since here 

 it appears in section in the form of a dorsally directed fold, it is evident that it 

 goes still further anteriorly and enters the column between the ventral processes 

 of the chambered organ. 



After the vestibule has completed its migration to the posterior end of the 

 animal and the oral ccelome has become horizontal the changes in the right or 

 aboral ccelome cease for a time. 



On the ventral side it has extended outward over the stomach and the 

 posterior gut section, and also incloses the greater part of the esophagus. Between 

 radius 4 and radius 5 it has grown far posteriorly, and there on some places covers 



