502 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



ectoderm soon loses its distinctness from the mesoderm, the ciliated bands degen- 

 erate, and histolysis, setting inward from the ectoderm, rapidly spreads to all the 

 tissues, and results in the obliteration of almost all the previously existing histo- 

 logical differentiation. The details of this histolysis are extremely difficult to 

 follow, and both in its mode and degree of action and in the relative tunes at 

 which it affects different organs it appears to vary considerably. 



Bury notes that the number of columnals in the free-swimming larva is 

 variable, but by the time the larva fixes itself from 13 to 15 are usually present, 

 and this number is maintained throughout the pentacrinoid stage. 



Prof. A. Russo, taking exception to Seeliger's interpretation of the conditions 

 in Antedon adriatica, states that the parietal sac is from the beginning in com- 

 munication with the stone canal, and that at first, as in all the echinoderms, it alone 

 communicates with the exterior, opening on the back of the larva near the fourth 

 ciliated band. In the course of development this primitive opening is obliterated, 

 so that in the fixed larva a few days old the madreporic system is composed of an 

 internal sand canal which carries at its extremity, turned inwardly, a vesicle 

 which is the parietal sack. Communication between this system and the exterior 

 is reestablished later, when the column has become elongated and the oral valves 

 have formed. It comes about through a secondary and independent formation 

 from the true water vascular system that is, by means of a canal of ectodermic 

 origin which ultimately opens into the parietal sack. 



Russo noticed that below the point which corresponds to the opening of the 

 secondary ectodermal canal leading into the parietal sack a primitive gonad is 

 formed, which after a short time atrophies. 



ANTEDON BIFIDA. 



Figs. 1176-1181, pi. 28, 1186-1189, pi. 29, 1191-1193, pi. 30, 1195, pi. 31, 1201-1203, pi. 32, 



1204-1206, pi. 33. 



Our knowledge of the embryology of Antedon ~bifida is based upon the work 

 of Busch (1849, 1851), Sir C. Wyville Thomson (1865), and Prof. W. B. Car- 

 penter (1866), all of which was done before the inception of modern methods of 

 technique. The results of their studies, therefore, are not strictly comparable with 

 the elaborately detailed investigations of Seeliger, who was able to avail himself 

 of all the latest refinements. But certain important features stand out which are 

 not dependent upon differences in methods. 



According to Sir Wyville Thomson the eggs of this species when fully grown 

 measure about 0.5 mm. in diameter. 



The formation of the polar bodies may be very easily followed. A very distinct 

 globule about half the diameter of the germinal vescicle, with an obscure nucleus, 

 passes out of the egg. In all cases in which Thomson observed it, it was accom- 

 panied by two or three minute rounded granular masses. During the earlier 

 stages of segmentation the polar bodies remain perfectly distinct, but toward the 

 beginning of gastrulation it becomes difficult to differentiate them from the cells 

 of the embryo. 



