.110 BULLETIN OF THE 



posed of a slightly opaque greenish centre, vt, the diameter of which is 

 two-thirds that of the egg-capsule, and a superficial transparent layer t, 

 which may be a thickened envelope, part or the whole of which later 

 becomes a vitelline membrane. Both these elements of the ova are 

 affected by segmentation. 



The transparent plasmic envelope of the vitellus of Ophiopholis is 

 thought to be identical with a similar layer described by others in star- 

 fishes and sea-urchins. Selenka,* for example, has described a similar 

 layer in Toxopneustes variegatus, and the question of the origin and fate 

 of the vitelline membrane (?) has been discussed by Giard, t Fol, % 

 Perez § and Selenka. || The discussion of the intricate complications of 

 the question which the origin and fate of the cortical layer of the Echino- 

 derm egg necessitate, must be passed over at present, as most of the 

 phenomena considered by the above-mentioned authors antedate a stage 

 of the Echinoderm egg corresponding with the youngest Ophiopholis 

 which I have studied. The observations here recorded are supposed to 

 have a value in indicating the existence of the cortical transparent layer 

 in the Ophiurans where, possibly with the exception of Amphiura, it has 

 been overlooked by other embryologists. 



The outer or superficial layer of the yolk is believed to be the same as 

 the " Structurlosen geblichen HUlle," described by Metschnikoff (op. cit. 

 p. 14), in Amphiura. The same layer is thought to be figured by him in 

 PI. III. figs. 3-6, mv. as the " Dotterhaut." 



While the outer membrane of the Amphiura egg is conspicuous in 

 Metschnikoff's figures of the youngest eggs, in older stages it suddenly 

 disappears. The same thing occurs in the capsule of the Ophiopholis 

 egg, which leads me to suspect that they are the same structures. 



According to Apostolides ^ a part of the protoplasm seems to condense 



* Beobachtungen uoer die Befruchtung und Theilung des Eies von Toxopneus- 

 tes variegatus. Sitsungsb. d. phys.-med. Gesells. z. Erlangen. X. pp. 1-7. Zoologische 

 Studien. I. Befruchtung des Eies von Toxopneustes variegatus. Leipzig, 1878. 



t Compt. Rend. LXXXIV. 7 (Trans.) Ann. Mag. Nat. Hist. [4] XLX. 113, pp. 434- 

 436. 



t Compt Rend. LXXXIV. No. 14 ; id. LXXXV. No. 4 ; id. No. 14. 



§ Op. cit. 



II Op. cit. The external transparent region mentioned above may correspond 

 with the "superficial hyaline membrane" described by Fol (Ann. Mag. Nat. 

 Hist. [4] XX.) over the vitellus of the starfish ovum. See also Perez, Sur la 

 Fecundation de I'CEuf chez I'Oursin. Compt. Rend. Acad. Sci., LXXXIV. p. 620; 

 LXXXV. p. 353. Ann. Mag. Nat. Hist. [4] XX. pp. 156-158. 



IT Op. cit. 



