MONOGRAPH OF THE EXISTING CKINOIDS. 465 



shaped mesenchyme cells increase the length of their processes, which then unite 

 with those of the ectoderm cells. When it is considered that individual ectoderm 

 elements of varied form pass down beneath the processes of the others, and that 

 individual mesenchyme cells pass upward to the same level, it is obvious that a 

 sharp line of demarcation between the ectoderm and mesoderm can no longer be 

 drawn, and that in regard to numerous cells it is impossible to determine the layer 

 from which they have arisen. 



In many cases Seeligcr found the histological character of the ectoderm at 

 the posterior end of the body, where it forms the closed roof of the vestibule, some- 

 what different from that covering the remaining portions of the body surface. 



Under the sharply defined external layer is a layer of plasma containing 

 numerous nuclei; cell borders are only to be made out vaguely in a few places. 

 Toward the body cavity a few pinlike plasmatic processes extend, and between 

 them are a few subepithelial ectoderm cells which are united with the superficial 

 cells by broad plasmatic bridges. 



The central portion of the flattened ectoderm wall of the vestibule has already 

 approached the outer body wall, indicating the position of the later rupture. 

 In younger larvae this region is often one of especial thiclcness, including an 

 abundant connective tissue which later is in part resorbed. 



Immediatelj' after the attachment the whole region of the apical pit imdergoes 

 the same alterations as the rest of the ectodermal epithelium, and becomes in 

 every way similar to it. The tuft of cilia vanishes; with the appearance of the 

 intermediate substance the sense and supporting cells lose their characteristic 

 forms, and the ganglion cells also can no longer be recognized as such. The thick 

 layer of nerve fibrillae disappears, and the membrane separating it from the body 

 cavity is no longer demonstrable. There is no further trace of the longitudinal 

 nerve cords running on either side of the vestibular invagination. 



After fixation the region of the attachment pit quickly broadens to a disklike 

 structure, which in the older larvae may acquire a very considerable circumference. 

 Over the surface of this disk there is a thin layer of a viscous secretion arising 

 from the gland cells which accomplishes the fixation. The surface itself is quite 

 smooth, without processes which might penetrate a foreign substance. 



The terminal stem plate has now moved down directly on to the place of 

 attachment, and in some places is coveered only by a single, sometimes very thin, 

 layer of cells. Some of these cells stretch out coarser or finer processes through 

 the holes in the plate. The nuclei can be easily recognized in the granular plasma ; 

 they show the chromatin in varying amounts and arranged in different ways. 



VESTIBULE. 



Normally very soon after fixation, but exceptionally at the end of a long free- 

 swimming period, the vestibular invagination becomes entirely closed off from 

 the ectodermal epithelium. This i^rocess takes place very quickly, within a few 

 hours. 



