ISOMETRA VIVIPARA. 35 



vesicles, the right and left enterocoel vesicle. In the stage represented in 

 plate xiv, figures 8 to 10, the two enterocoel vesicles are still united by a 

 narrow transverse canal. (The sections are obliquely directed, which has 

 necessitated representing three of the sections; combined they correspond 

 to that represented in figure 7.) A sagittal, longitudinal section of a corre- 

 sponding stage is represented in plate xv, figure 1. It is important to notice 

 that there is no trace of the downward prolongations from the anterior vesicle, 

 which in Antedon embrace the narrow transverse canal that connects the 

 two enteroccelic vesicles. In this regard Isometra is in accordance with 

 Tropiomelra. The lumen of the entero-hydroccel is very large. 



The formation of the hydroccel could not be made out in all details. Prob- 

 ably the upper lobe in plate xiv, figure 6, is the first indication of the hydro- 

 ccel, and the faint constriction shown in plate xiv, figure 7, may possibly 

 mean that the whole anterior part of the entero-hydroccel is destined to 

 form the hydroccel. The question is not very important, the main thing 

 being that the hydroccel is formed from the anterior vesicle, as in Antedon 

 and Tropiometra. 



The formation of the parietal canal could not be made out in all 

 details, but from the stage represented in plate xvi, figure 1, it is evident 

 that it proceeds in the usual way, being formed as a constriction from the 

 hydroccel vesicle. 



The ectoderm (plate xiv, figures G to 11) consists of high epithelial cells, the 

 nuclei lying mainly in the middle, forming a more or less distinct layer. The 

 entoderm also consists of cylindrical epithelial cells, with the nucleus mostly 

 at the base and the whole space of the cell filled with yolk spherules (plate 

 xv, figure 2). Also the enteroccelic vesicles show the same epithelial structure 

 (plate xiv, figure 7). The mesenchyme cells have become discernible in the 

 stage when the division of the archenteron begins (plate xiv, figure 6). 

 Whether they originate from the entodermic epithelial cells or from the nuclei 

 originally lying spread in the yolk-mass can not be ascertained, but from the 

 fact that they lie at first mainly at the upper end of the archenteron and do 

 not nearly fill out the blastoccel cavity (which has become very large compared 

 with the preceding stage, plate xiv, figure 4), the evidence is decidedly for their 

 originating, as in Antedon and Tropiometra, from the entoderm epithelium. 

 Gradually the blastoccel cavity is completely filled out by the mesenchyme 

 cells, as seen very plainly by a comparison of plate xiv, figure 6, with plate 

 xv, figure 1. In figs. 8 to 10, plate xiv, they have filled the blastoccel cavity 

 completely. They are from the first filled with yolk spherules, like all the 

 other cells, both of ectoderm, entoderm, and hydroccel (plate xix, figure 11); 

 by and by these become less distinct, and the yolk spherules instead collect 

 into round groups which lie scattered irregularly, mainly in the mesenchyme. 

 The nuclei of ectoderm, entoderm, and mesenchyme are not distinctly 

 different in size. 



