262 BULLETIN OF THE 



ist eine einschichtige Lage von Flimraerepitliel. Dasselbe erscheint bei 

 durchfallendein Lichte braunlichgelb, bei auflallendeni Lichte spangriiu 

 gefiirbt. Der Luftsack selbst ist rings von den bellen Zellen des Nah- 

 rungsdotters umgeben, und steht nur an seinem proximalen Ende (der 

 Abschniirungsstellc) in Beriihrung mit der Wand der Polypitenbasis, 

 welche daselbst in das Deckstiick libergeht." 



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 Deckstuckcs, auf der Flache, die ich als Riicken- 

 flache bezeichne, eine locale Ectodermverdickung, welche als erste Spur 

 des Luftapparates angedeutet werden muss. Am sechsten Tage hat sie 

 die Form eines halbkugeligen Ktirpers angenommen, der unter der iius- 

 seren Ectodermbedeckung und in der Nahe des einstweilen noch localeu 

 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 layei'. The subsequent growth of the float will b*e 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 

 {mh.), 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 (1). The forming cov- 

 ering-scales called serrated hydrophyllia {ser. hyjih.) have a slightly I'cd 

 color. 



The primitive cavity {c.p. I.) 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. 



