"WOODWAUD : LA.KVA OF THE ETIROPEAN OYSTER. 299 



stain to a yellowish red with htcmatoxylin, while the epiblastic 

 cellular structures stain much bluer : thus one can readily distinguish 

 the muscular tissue from the more cellular tissue composing the 

 other organs. 



Just in front of the anus, and directly connected with the epihlast, is 

 a rounded mass of cells (Fig. 1, o.g.), figured as such by Horst {loc. cit., 

 Fig. 12) and Huxley {loc. cit., Fig. 1). The latter did not delineate 

 the cellular nature in this or any other organ, but a reference to his 

 original drawing shows that he did not regard this as a muscular 

 structure, for he has coloured all such structures red, while this is 

 coloured brown with the epiblast. 



A very careful examination of this structure iinder a higher power 

 shows that it does not reach the under surface of either valve, but is a 

 knob-like mass of epiblast extending into the body tissue. Now it is 

 well known that muscles arise from the mesoblast, and not directly 

 from the epiblast. At the stage now under consideration the cells 

 constituting the mesoblast are scattered through the body, and not 

 as yet specially aggregated in the position of the future posterior 

 adductor muscle. Further, the posterior velar and the pedal retractor 

 muscles pass on either side of this mass, or, when viewed fi-om the 

 side (Fig. 1), above and below it, thus cutting it off completely from 

 the shell. 



This epiblastic cellular mass, in all probability, represents the 

 " anlage " of the so-called parieto-splanchnic (olfactory) ganglia, just 

 appearing. It is not the posterior adductor muscle, as stated by Webb, 

 and his figure does not represent its true relationships. The fibres 

 represented in his fig. 2 at " p. add." do not exist. 



I conclude, then, that the di-myarian stage of Ostrea edulis has not 

 yet been discovered, although it is undoubtedly passed through. So 

 far, the specimens examined have been too young, and it will be 

 probably found, as Jackson found it, in a newly attached oyster, not 

 in an embryo still contained in the brood chamber of the parent. 



EXPLANATION OF PLATE XX. 



List of Abbreviations. 



a.a. anterior adductor muscle. e.g. P olfactory ganglion. 



a.v.r. anterior velar retractor. p.c. pallial chamber. 



e.g. cerebral ganglion. p.g. vestigial pedal ganglion. 



h.d. digestive diverticula. p.v.r. posterior velar retractor. 



f. foot. St. stomach. 



i. intestine. v. velum. 



m. mouth. 



Figures all drawn with camera lucida. X 400. 



Fig. 1. Side view of " black spat" stage of the EiU'opean oyster, from a specimen 

 prepared by Van Horst. 

 ,, 2. Antero- dorsal view of another specimen of the same age. 

 ,, 3. Antero- ventral aspect of a contracted specimen. 



