262 BULLETIN OF THE 
ist eine einschichtige Lage von Flimmerepithel. Dasselbe erscheint bei 
durchfallendem Lichte braéunlichgelb, bei auffallendem Lichte spangriin 
gefirbt. Der Luftsack selbst ist rings von den hellen Zellen des Nah- 
rungsdotters umgeben, und steht nur an seinem proximalen Ende (der 
Abschniirungsstelle) in Beriihrung mit der Wand der Polypitenbasis, 
welche daselbst in das Deckstiick iibergeht.” 
In the genus Agalma Metschnikoff thus describes the appearance of 
the float on the fifth day. He says:* ‘Zu gleicher Zeit bemerken wir 
dicht unterhalb des Deckstiickes, auf der Fliache, die ich als Riicken- 
fliche bezeichne, eine locale Ectodermverdickung, welche als erste Spur 
des Luftapparates angedeutet werden muss. Am sechsten Tage hat sie 
die Form eines halbkugeligen Korpers angenommen, der unter der ius- 
seren Ectodermbedeckung und in der Nahe des einstweilen noch localen 
Entoderms seine Lage findet.” The origin of the float in Agalma elegans 
resembles more closely that of Agalma Sarsii than that of Crystallodes. 
It arises as a simple epiblastic elevation of the yolk surface, not far from 
the proximal side of the hydrophyllium. That elevation is primarily of 
epiblast, but later the hypoblast may also enter into its formation. As 
the float grows older, the bud diminishes in size, thickening inward, and 
a separation of the hypoblast from the epiblast takes place, which is 
filled by an intermediate body, either thickened epiblast or the interme- 
diate or middle layer. The subsequent growth of the float will be seen 
in descriptions of later stages of the primitive larva. 
In Fig. 13 we find that the primitive hydrophyllium has increased 
very much in size, while in the progress of that growth the distinction 
between the proximal and distal edges of the disk which we have earlier 
detected are still maintained. The great body of the scale is gelatinous, 
the mass of which is formed by an enormous growth of a middle layer 
(mb.), which lies between epiblast and hypoblast. The relative thick- 
ness of the epiblast has greatly diminished. It is still ciliated and easily 
distinguished from the other layers when seen in profile and along the 
rim of the hydrophyllium, while scattered over the surface of the scale 
appear the small epiblastic structures or nuclei(?). The forming cov- 
ering-scales called serrated hydrophyllia (ser. hyph.) have a slightly red 
color. 
The primitive cavity (c. p. /.) lined with hypoblastic cells which have 
a distinct yellow color has risen with the growth of the bell, and extends 
towards the distal rim of the hydrophyllium. The edges of the disk 
are free, the hydrophyllium fitting over the egg like a helmet, the visor 
* Op. cit., p. 50. 
