. MONOGRAPH OF THE EXISTING CRINOIDS. 441 



circles of different diameters, and in sections cut parallel to this axis as a pair of 

 ellipses, with the longer axes parallel. 



The more the separation of the two sections progresses the more strongly comes 

 out the contrast between them. First the posterior section becomes more and more 

 strongly compressed dorsoventrally, and laterally elongated parallel to the blasto- 

 pore slit, so that it assumes an almost tubular foiTn. The anterior section then 

 extends itself ventrally and dorsally partially around the tube-like posterior por- 

 tion, so that in section the lumen of the former appears as a narrow crescent stretch- 

 ing dorsally and ventrally just beyond tlie Ijroadly elliptical lumen of the latter, 

 which runs from riglit to left. 



At this stage there is no clearly marked bilateral structure which can be 

 followed with certainty through all the following changes to the fixed pentacrinoid. 

 The major axis, equivalent to the longitudinal axis, can be determined in the 

 earliest division stages. The tube-like posterior section of the primitive gut mns 

 from right to left. The ventral and dorsal sides are at first almost identical, except 

 tJiat on the ventral the entodermal sack reaches nearer the ectoderm than on the 

 other. In the same way the riglit and left sides are never exactly the same. Very 

 soon, when the dorsal and ventral sacks become differentiated, symmetry of the 

 bilateral type becomes again established. 



During the division of the primitive gut the region from which the mesen- 

 chyme cells are given off becomes more restricted and finally reduced to a small 

 area of the anterior wall of the upper entoderm sack in the immediate neighbor- 

 hood of the major axis, mesenchyme cells at other places appearing only sporadi- 

 cally. It is therefore to be assumed that the mesoderm cells which push in 

 between the ectoderm and the posterior sack have arisen through the division 

 of those earlier formed. 



The form and size of the cells in the three germ layers, which were already 

 differentiated in the 2G-hour embryo, have meanwhile become appreciably altered. 

 All the cells have increased in number and decreased in size, and their yolk 

 elements have diminished and in part been already entirely consumed. 



In the ectoderm there are found, as in the earlier stages, globular elements 

 in process of division among the high prismatic cells, the nuclei of which lie now 

 in the inner ends. But with the migration of the nuclei into the inner ends of 

 the cells the distribution of tliese globular elements undergoes a change, for 

 they are now not confined to the outer portion of the ectoderm but occur, though 

 in small numbers, also on the inner surface toward the body cavity. 



The walls of the coelomic (posterior) sack, like tho.se of all the epithelial 

 structures of the embryo, are of one layer of cells, but the shape of the individual 

 cells varies considerably. Here again appear the globular cells representing the 

 prismatic cells in process of division, though cell reduplication also occurs by the 

 direct division of the prismatic cells themselves. 



In the anterior sack the differences in form of the individual cells are more 

 considerable than in the posterior. Here also cell division occurs b<ith directly 



