No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 



59 



ventral surface is a shallow indentation, the floor of which is 

 formed of small, closely appressed cells (oral plate ; " Mittel- 

 schild," Flemming). Just beneath the anterior angle of the 

 valves of the shell is the opening of the thread-gland, surrounded 



Fig. 3. — Fifty-cell stage of Unio 

 from the right side. 



Gastrula of Unio. 



by five large cells (Figs. 80 and 81). The median line, from the 

 opening of the thread-gland to the oral plate, is occupied by 

 long cells drawn out in an antero-posterior direction. These 

 are the suture-cells (Nahtzellen of Flemming), to which more 

 attention will be directed in the next section. Below the pos- 

 terior angle of the valves of the shell in the median line is a 

 plate of ciliated cells ; this I have called the ventral plate 

 (Wimperschild of Flemming). These cells extend forward, and 

 stop suddenly at the larval mantle cells (the columnar cells of 

 the ventral surface already referred 

 to). The place where the ventral / y — ^ 1 ^ 

 plate meets these cells is the anterior 

 limit of the blastopore. Lying with- 

 in the ectoderm at this spot is the 

 entodermic sac (Vorderwulst of 

 Flemming), with mesodermic cells 

 attached (Figs. 73, TT, and 'jZ). The 

 teloblasts of the mesoderm have now 

 moved from their former position in 

 the angle of shell-gland and ventral plate to just behind the ento- 

 dermic sac. Two stout wings of cells pass from here to the 

 lateral ectodermic walls (PI. V, Fig. jy) (Seitenflugel of Flem- 

 ming). Within the primary body cavity are numerous elongated 



Fig. 5. — Young Larva of Unio. 



