MONOGRAPH OF THE EXISTING CRINOIDS. 473 



the left coelome exteriorly, although this also in the same region extends further 

 posteriorly than elsewhere. 



In the individual the form of the body cavity is determined through tlie 

 progressive development of the intestinal canal ; with the exception of the posterior 

 side on which lies the oral coelome tliis remains entirely inclosed in the epithelium 

 of the right coelome. 



Wlien the stomach and gut are excessively extended by an anusually large 

 inner mass, the outer and inner ccelomic layers are closely pressed against each 

 other, and there is no body cavity to be seen. On the dorsal side this is frequently 

 observed; but ventrally there alwaj's persists a more or less extensive space between 

 the stomach and lateral extension of the gut where the originally left tip lies. 

 This part of the i-ight coelome grows posteriorly indej>endently so that it appears 

 on cross sections as a blind sack closed on all sides. 



The hinder section of the gut lies on the border of the oral ccelome in the 

 vertical mesentery, and so as a result of its enlargement the two layers of the 

 latter become spread apart, retaining their former close association only anteriorly. 

 It further comes about that the place of union between the gut and stomach 

 gradually narrows dorso-ventrally, and thus the blind end of the aboral ccelome 

 lying in this corner shoves itself further dorsally. So the gut remains in spite 

 of its increased length covered on all sides by coelomic epithelium. 



The region between esophagus, stomach, and middle gut is where the horizontal 

 mesentery vanishes, and both parts of the cuelome unite. 



It is the right ventral tip of the oral ccelome which breaks through into the 

 left portion of the aboral. The opening appears first at the ventral end near 

 the median plane and extends thence dorsalward toward the right. 



First the two layers of epithelium grow together; later in the center of this 

 region an extraordinarily fine slitlike cavity is formed, which widens rapidly, 

 while along its borders the two layers become completely fused, passing uninter- 

 ruptedly over into each other. Since before the union the walls of the two cavities 

 were histologically similar, it is now impossible to discover at what point the 

 union took place except on a circumscribed region on the inner side where, at 

 least at first, the extension of the axial organ characterizes the right coelome. 



At this stage the axial organ occupies the entire extent of the ventral side 

 of the inner wall of the aboral ccelome. In the most anterior part, near the cham- 

 bered organ, it lies as a solid cord of cells on the vertical mesentery. Further 

 posteriorly it becomes more prominent, standing out as a pleatlike thickening lying 

 on the gut. At the posterior end is the youngest developmental stage. There is no 

 evidence that mesenchyme cells take part in its formation. 



Under high magnification it can be easily seen how the flat ccelomic epithelium 

 juts out into the body cavity and the cells involved become richer in plasma and 

 divide, so that a several layered swelling results among the cells of which there 

 very soon appears a differentiation of the nuclei, some rapidly increasing in size 

 and becoming vesicular. 



