MONOGRAPH OF THE EXISTING CRINOIDS. 443 



of one layer of cells. In some embryos a few mesenchyme cells are given off 

 in the immediate vicinity of the major axis, while in others this process has 

 already ceased. 



From this central portion ventrally there stretches an extensive process, the 

 hydroccele, which at first maintains a broad connection with the remaining parts 

 of the anterior sack. 



The beginnings of the parietal canal can often, though not always, be deter- 

 mined at this stage. It first shows itself as a small evagination of the anterior 

 wall of the mesentero-hydrocoele sack lying anterior to the union between the two 

 ccelomic pockets on the border of the positions of the gut and water vessel. 



While in the cases described both sections of the primitive gut are entirely 

 cut off from each other, equally old and almost equally organized embryos can 

 be found in which a union between the crelomic and mesentero-hydrocoele portions 

 is demonstrable near the major axis. 



The primitive body cavity is now entirely filled with mesenchyme cells, which 

 have greatly increased in number. Most of them are approximately spherical, 

 but many are pear-shaped, while others exhibit a tendency to send out processes 

 and to become star-shaped. The varying amount of chromatin in the nuclei of 

 these cells is readily demonstrable. 



FORMATION OP CILIATED BANDS, LARVAL NERVOUS SYSTEM. AND ALTERATION IN POSITION OF INTERNAL ORGANS 



(60 TO 84 HOURS). 



During the third night very important changes take place in the embryos, 

 though the developmental advances are no longer as completely synchronous as in 

 the younger stages. Only in rare cases now does one find all the embryos on a 

 single mother at an equivalent developmental plane; usually the variations ex- 

 hibited are considerable. Thus on the fourth day there are found, in immediate 

 proximity to the older stages, others which have lagged behind 12 or 14 hours, so 

 that it is possible to study in the embryos from a single mother all the changes which 

 have taken place during the night. 



Externally the embryos of this age are without exception characterized by 

 an elongation along the chief axis, and by five transverse ciliated bands, of which 

 the foremost is interrupted in the ventral median line. The average length is 

 now 0.30 mm. 



The four posterior ciliated rings encircle the embryo like the bands of a cask, 

 but they no longer run quite parallel to each other. As a result of the appearance 

 of the vestibular invagination in the median plane ventrally the third band is arched 

 posteriorly, and the second is, to a somewhat greater degree, arched anteriorly. 

 The further the invagination extends the more the mid-ventral portions of these 

 two bows are displaced. 



Just before the displacement of the mid- ventral portion of the second and third 

 ciliated bands the entire ventral side of the embryo appears thickened and a little 

 flattened. Most of the cells are exceptionally long prismatic cells, which extend 

 uninterruptedly from the primary body cavity to the outer surface. Their nuclei 



