JOURNAL OF MICROSCOPICAL SCIENCE. 



EXPLANATION OF PLATE XXVII, 



Illustrating Dr, Horst's Paper on " The Development of 

 the European Oyster [Ostrea edulis, L.).'^ 



Fig. 1. — Au egg in segmeutatiou, consisting of a large hypoblastic 

 sphere and several epiblastic eells. 



Fig. 2. — Surface view of a later stage, witL two hypoblastic cells. 



Fig. 3. — Embryo viewed from the side, with begining invagination (gas- 

 trula). 



Fig. 4.— Optical section through a more advanced stage, with the hypo- 

 blastic invagination and the first apparition of the shell-gland — ec, epi- 

 blast ; eti, hypoblast; o, blastopore; sk, shell-gland. 



Fig. 5. — Au older embryo viewed from the side — v, foot. 



Fig. G. — Au optical section through the same stage — me, mesoblast ; 

 d, arclienteron. 



Fig. 7. — Stage one (iay older, viewed from the front side, with blas- 

 topore. 



Fig- 8. — Optical section through the same stage. 



Fig. 9. — Optical section through au embryo, one day older, with velum, 

 ston^ch and shell, s. 



Pig. 10. — A later stage, viewed from the side, with more develooed 

 • shell. 



Fig. 11 — -More advanced larva, with tlic epiblastic tliickoniug in the 

 midst of the velar area — a, anus ; e, intestine; m, stomach ; d, oesophagus ; 

 tp, cephalic thickening. 



Pig. 12. — Older larva with a double prteoral ring of cilia, nerve ganglion, 

 hepatic sacs and muscles — ds, dorsal longitudinal muscle ; vs, ventral lon- 

 gitudinal muscle; sp, adductor muscle; /, hepatic sac; mh, pallial cavity. 



