No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 5 



appeared, but, as if by common consent, the Lamellibranchiata 

 have been ignored. The eggs of most marine lamellibranchs 

 are exceedingly small and difficult to handle. This has prob- 

 ably deterred others, as it has me, from studying them, or has 

 made the results of such studies as have appeared {e.g., on the 

 oyster) of greater economic than scientific worth. 



My object in studying the Unionidae has been to fill in, so 

 far as I could, the two gaps indicated above, i.e., first, in our 

 knowledge of the derivation of the germinal layers in the 

 Unionidae ; and, second, in our knowledge of the early devel- 

 opment of the Lamellibranchiata. I have followed in detail 

 the lineage of the cells as the best way of reaching conclusive 

 results as to the origin of the germinal layers of Vermes and 

 Mollusca. The first division of the work deals with the origin 

 of the germinal layers. In the second part I take up the 

 development of the gastrula in the glochidium. 



II. Cleavage of the Ovum and Derivation of the 

 Germinal Layers. 



(a) Natural History. 



The Unionidae carry their young in the external gills until 

 the completion of embryonic development, when they are fully 

 equipped for their temporary parasitic existence. The eggs 

 are fertilized, after extrusion, in the suprabranchial chamber 

 (Schierholz, No. 30, Rabl, No. 25), spermatozoa gaining access 

 with the respiratory current of water. They then pass along 

 the suprabranchial chamber of the internal gill to the cloaca, 

 where they pass over to the external suprabranchial chamber, 

 and moving along it anteriorly fill the interfoliar spaces from 

 one end of the gill to the other.^ In Unio the eggs adhere 

 to one another in the form of oval plates, which may contain 

 as many as 1000 eggs^ each. The eggs composing each plate 

 are bound together by a sort of cement, which gradually dis- 



1 I noticed that, in a species of Anodonta, which is very common near Worces- 

 ter, Mass., only the posterior half or two-thirds of the external gills is used for 

 the reception and development of the ova. 



2 A rough calculation. 



