PRELIMINARY ACCOUNT OF THE EMBRYOLOGY 

 OF UNIO COMPLANATA. 



F. R. LILLIE. 



The following paper is preliminary to a more extended one, 

 which I hope to publish soon. In the complete paper I shall 

 advance the proof of the statements made here. 



In carrying out the work two main objects have been kept 

 in mind: — 



Firstly to settle definitely the question of the origin of the 

 germ layers in the Unionidae, and 



Secondly to extend the cytogenetic method in embryological 

 research to the class of the Lamellibranchs ; a thing which 

 has not hitherto been attempted in a very minute fashion. In 

 the attempt to attain to these two main objects of the work, 

 other problems of greater or less interest have presented them- 

 selves. 



Rabl 1 made a fundamental error in the interpretation of the 

 shell-gland as the invagination of the entoderm. Balfour, 

 while provisionally accepting Rabl's account, nevertheless calls 

 attention in his text-book to the strong inherent improbability 

 of the dorsal position of the blastopore. In 1891 Goette^ 

 published an account of his observations on the formation of 

 the entoderm in Anodonta, and figured its invagination at the 

 very region to which Rabl described it as wandering, after its 

 supposed formation dorsally. In Unio the invagination takes 

 place in the same region as described for Anodonta by Goette, 

 but in an earlier stage of development. 



Although Goette cleared up to a certain extent the question 

 of the origin of the entoderm, he did not do so for the meso- 

 derm; and for want of any contradiction, based on observation 

 of the facts, Rabl's account of the origin of the mesoderm 



1 Rabl, Ueber Entw. der Malermuschel. Jen. Zeitschr. f. Naturwiss. Bd. X. 

 pp. 310-393. 



2 Goette, Bemerkungen iiber die embr. Entw. von A. piscinalis. Zeitschr. f. wiss. 

 Zool. lii. 



569 



