50 



LAMELLIBRANCHIA UNIONIDAE. 



The ontogeny of the Unionidae has been studied by a number of 

 zoologists. FLEMMING, EABL, GOETTE and SCHIEBHOLZ have in- 

 vestigated their embryonic development, while the later stages of 

 their development, which were examined by FOEEL (No. 13), LEYDIG 

 (No. 32), BEAUN (Nos. 4 and 5), BALFOUE, F. SCHMIDT (No. 50) 

 and others, have recently been reinvestigated by SCHIERHOLZ and 

 GOETTE. 



[Still more recently, LILLIE (No. III.) has reinvestigated the entire 

 course of development in Unio complanata, paying special attention, 

 however, to the cell -lineage.] 



mes 



mes 



A. Development of the Early Stage. 



It has already been mentioned that the Unionidae show an in- 

 vagination-gastrula (p. 27, etc.) and that, before the latter develops, 

 large mesoderm-cells bud off from the wall of the blastula and enter 

 the cleavage-cavity. Before the formation of the very insignifi- 

 cant archenteron 

 which, like that of 

 other Lamellibranchs, 

 is derived from the 

 macromeres resulting 

 from the unequal 

 cleavage, a depression 

 appears on the blastula 

 and deepens more and 

 more (Fig. 22, sd). 

 This depression is 

 formed by large cells 

 which are granular 

 and therefore appear 

 dark, and its whole 

 form is such that we 

 can easily understand 

 why it was long mis- 

 taken for the archen- 

 teron (p. 27). This depression, however, does not occur on the ventral 

 side of the embryo, but upon its dorsal surface ; it gradually flattens 

 out again and above it the shell-integument appears (Figs. 23 A-C, 24 

 A). This structure is therefore, as GOETTE proved, the shell-gland.* 



FlG. 22. Embryo of Anodonta in the vitelline mem- 

 brane (after SCHIEBHOLZ) ; ent, entoderm-rudiment 

 (archenteron) ; eh, vitelline membrane ; m, micropyle; 

 mes, mesoderm-cells, some of which have turned into 

 muscle-cells ; rk, polar bodies ; sd, shell-gland ; sz, 

 lateral cells ; w, posterior ciliated area [ventral plate]. 



* [According to LILLIE (No. III.) the entomeres which eventually become 

 invaginated to form the archenteron are derived from all the four cleavage- 



