4 LILLIE. [Vol. X. 



the region of the future dorsal surface, which freed themselves 

 after invagination and moved to the anterior end (really to the 

 posterior end) of the body, where they took up their definite 

 position. Believing, as he did, that the dorsal invagination was 

 the primitive intestine, it was natural to suppose that the telo- 

 blasts of the mesoderm, which lie just beneath the posterior 

 end of this invagination, were derived from the cells composing 

 its wall. There was a threefold error here: first, in mis- 

 taking the posterior for the anterior end of the larva; ^ second, 

 in interpreting the dorsal invagination, really the shell-gland, 

 as the archenteron ; and third, in deriving the teloblasts of 

 the mesoderm from the cells of the dorsal invagination. The 

 first error was corrected by Schierholz (Nos. 35 and 36), and 

 the second by Goette (No. 29), In my preliminary account I 

 have already pointed out the true source of origin of the telo- 

 blasts of the mesoderm. 



Not since the time of Rabl's paper has any work been pub- 

 lished dealing in a thorough way with the whole embryonic 

 development of the glochidium larva. Schierholz's paper, it 

 is true, covers the whole period of development, but does not 

 adduce much that is new on the embryonic portion other than 

 the correct orientation of the larva. Goette's paper confines 

 itself to the formation of the entoderm in Anodonta. Other 

 papers since the time of Rabl have dealt only with the postem- 

 bryonic development, thus beginning where my work ends, and 

 hence not calling for remarks here. It would seem, then, as 

 though it were time for another work on the embryonic devel- 

 opment. 



Great advances have been made within the last fifteen years 

 in the adult morphology of the Lamellibranchiata ; but since 

 the time of Hatschek's paper on Teredo, no corresponding 

 advances have been made in the embryology of these forms. 

 In saying this I do not mean to belittle such works as those 

 of Brooks, Jackson and Horst on the oyster or Ziegler and 

 Stauffacher on Cyclas. During the same time many impor- 

 tant embryological works on other classes of Mollusca have 



1 Flemming made the same mistake ; Forel, who in point of time preceded 

 both Flemming and Rabl, was right. 



