306 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



The ciliation is still quite uniform, and there is no indication as yet of the ciliated 

 bands. The blastopore closes immediately after the liberation. In embryos ten hours 

 old the apical tuft of cilia is distinct, but the ciliated bands do not begin to appear until 

 the embryo is twelve hours old. 



The ectoderm, which in embryos six hours old is a typical epithelium with the 

 nuclei arranged fairly regularly in a single series at the basal end of the cells, is con- 

 siderably thickened with the nuclei arranged pluri serially. It is, however, still 

 distinctly a single layered epithelium. At the anterior end under the apical tuft it is 

 more or less thickened, this part corresponding to the apical pit of Antedon, but it is 

 never so conspicuous as in that form. In embryos eight hours old the archenteron is 

 nearly separated into an upper and lower part through a median constriction. The 

 blastocoel cavity is completely filled by mesoblast cells; also at the oral end, below the 

 archenteron, a group of mesenchyme cells has made its appearance; that these are 

 derived from the lower end of the archenteron seems beyond doubt. 



The complete separation of the archenteron into an upper part, the entero- 

 hydrocoel (the mesentero-hydrocoel of Bury and Seeliger), and a lower part, the 

 coelomic vesicle, may take place at an age of only ten hours, though in other cases the 

 two parts may still be in wide connection at this age. 



In embryos 12 hours old the lower part of the coelomic vesicle begins to form two 

 lobes extending somewhat forward and connected by a narrower part. These two 

 lobes represent the right and left coelom. 



The anterior part, destined to form the intestine and the hydrocoel (the mesentero- 

 hydrocoel of Bury and Seeliger) forms a pouch which is doubtless the rudiment of the 

 hydrocoel. Although its constriction from the entoderm could not be followed, the 

 comparison with what obtains in Antedon, as represented by Seeliger, leaves hardly 

 any doubt that this is really the first trace of the hydrocoel. On the other hand 

 Mortensen says he would not venture to maintain that the pouch seen in a correspond- 

 ing place in an embryo only ten hours old and with the archenteron still undivided is 

 really the same thing, though it might not seem improbable. 



The entero-hydrocoel is a simple vesicle, with no posterior prolongations to embrace 

 the narrow middle part of the coelomic vesicle. The relation between the two primary 

 vesicles is thus quite simple, and the complicated structure that occurs in Antedon 

 is seen to have no general value hi the developmental history of crinoids. 



Concerning the histological character of the two entodermic vesicles it need only be 

 stated that they consist of a simple rather low epithelium, distinctly lower than the 

 ectoderm. The formation of mesenchyme cells appears to continue until about the 

 time when the separation of the two entodermic vesicles takes place ; but already at the 

 age of eight hours the blastocoel cavity may be completely filled by the mesenchyme 

 cells. The nuclei of the entoderm and mesenchyme cells are generally distinctly 

 larger than those of the ectoderm, a feature that may be observed throughout the 

 embryonic development. 



In embryos 16 hours old the ciliated bands are fully formed. There are only four 

 of these, not five as in Antedon adriatica and A. mediterranea. Sir Wyville Thomson 

 figured only four ciliated bands in A. bifida, so that it would appear that even within 

 the same genus the number of the ciliated bands may be variable. However, Morten- 



