28 



LAMELLIBBANCHIA. 



which latter only then commence to divide. A cavity then appears to arise 

 between the ectoderm and the entoderm, and stages occur exactly resembling 

 Fig. 14 B. In Teredo, the separation of the two primary germ-layers and the 

 increase of the entoderm-cells takes place at later stages (Figs. 12 and 15, 

 pp. 26 and 31). 



During the act of gastrulation (Teredo, Unionidae) or even before 

 it commences (Cyclas), the rudiment of the mesoderm appears in the 

 embryo. In the epibolic gastrula of Teredo, there are two large cells 

 which, according to HATSCHEK, are to be traced back to the macro- 



-mes 



FIG. 14. A-E, various stages of development of the Oyster (^4 of Ostrea virginiana 

 after BROOKS, B-E, of Ostrea edulis after HORST). a, anus ; U, blastopore ; ra, 

 mouth ; ma, stomach ; mes, mesoderm-cells ; rk, polar bodies ; s, shell (in D, un- 

 paired embryonic shell-rudiment) ; sd, shell-gland ; sm, the anterior adductor muscle ; 

 w, pre-oral ciliated ring. 



meres, lying symmetrically to the median plane at the posterior 

 edge of the blastopore (Fig. 12 A and B). They are soon grown 

 round by ectoderm and are thus drawn into the interior of the 

 embryo (Fig. 12 C). In Ostrea edulis, corresponding cells are 

 found in a similar position (Fig 14 C), and conditions similar on the 

 whole are also found in Cyclas.* These two cells have been assumed 

 to be primitive mesoderm-cells [mesodermal teloblasts] homologous to 



* [In Cyclas, after the macromere has given origin to the last micromere 

 {about the 30-celled stage), it divides into two cells of equal size, from each of 

 which a large cell segments off into the cleavage-cavity. These are the two 

 primary mesoderm-cells. The two small remnants of the macromeres form 

 the entoderm (STAUFFACHBR, No. VI). ED.] 



