10 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



form. In embryos 8 hours old the archentcron is nearly separated into an 

 upper and lower part through a median constriction (plate ii, figure 5). 

 The blastoccel 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 they 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," and a lower part, the coelomic vesicle, may take place at an age 

 of only 10 hours (plate ii, figure 6), though in other cases the two parts may 

 still be in wide connection at this age (plate ii, figure 7). 



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 (plate iii, figures 2 and 4; plate ii, figure 9); these two lobes represent 

 the right and left ccelom. In the section of an embryo 12 hours old, repre- 

 sented in plate iii, figure 2, is still seen the last trace of the connection 

 between the anterior and the posterior part of the archenteron. 



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

 " mesentero-hydroccel " of Bury and Seeliger), forms a pouch (plate iii, 

 figure 3), 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 Seehger, leaves hardly any 

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

 I would not venture to maintain that the pouch seen in a corresponding 

 place in plate ii, figure 7, from an embryo only 10 hours old and with the 

 archenteron still undivided, is really the same thing, though it might not 

 seem improbable. 



The entero-hydrococl 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 which occurs in Antedon is seen to have no general value in the 

 developmental history of Crinoids. 



Concerning the histological character of the two cntodermic vesicles, 

 it need only be stated that they consist of a simple, rather low epithelium, 

 distinctly lower than the ectoderm (plate ii, figure 7; plate iii, figure 4). 

 The formation of mesenchyme cells appears to continue until about the 

 time when the separation of the two entodermic vesicles takes place (plate ii, 

 figures 5 and 7) ; but already at the age of 8 hours the blastoccel cavity may 

 be completely filled up by the mesenchyme cells. The nuclei of the ento- 

 derm and mesenchyme cells are generally distinctly larger than those of 

 the ectoderm, a feature which may be observed throughout the embryonic 

 development. 



" I prefer to use this designation instead of the name "mesentero-hydrocoel" used by Bury and 

 Seeliger. 



