266 LAMELLIBRANCHIATA. 



The single large cell next divides into two, and then four, and finally into 

 about ten to fifteen cells. These cells form an especial area of more granular 

 cells than the other cells of the blastosphere. Most of them are nearly of 

 the same size, but two of them (according to Rabl), in contact with each 

 other, but placed on the future right and left sides of the embryo, are con- 

 siderably larger than the remainder. These two cells soon pass into the 

 cavity of the blastosphere, while at the same time the area of granular cells 

 becomes flattened out, and then becomes involuted as a small sack with a 

 transversely elongated opening, which does not nearly fill up the cavity of 

 the blastosphere. This involuted sack is the archenteron. 



The two large cells, which lie in immediate contact with what, following 

 Rabl, I shall call the anterior lip of the blastopore, next bud off small cells, 

 which first form a layer covering the walls of the archenteron, but sub- 

 sequently develop into a network filling up the whole cavity of the primitive 

 blastosphere. The space between these cells is the primitive body cavity. 

 For a long time the two primitive mesoblast cells retain their preponderating 

 size 1 . At the hinder end of the body, and at the end opposite therefore 

 to the two mesoblast cells, are placed three especially large epiblast cells. 



In Anodonta and Unio tumidus there appears at this period a patch of 

 long cilia at the anterior end of the body. These cilia cause a rotation of 

 the embryo and would appear to be the velum. In Unio pictorum they 

 do not appear till much later. 



Immediately following this stage the changes in the embryo take place 

 with great rapidity. In the first place a special mass of mesoblast cells 

 appears at the hinder end of the archenteric sack ; and becoming elongated 

 transversely gives rise to the single adductor muscle. On the subsequent 

 formation of the. shell the muscle becomes inserted in its two valves. 

 The blastopore next becomes closed, and the small archenteron grows for- 

 wards till it meets the epiblast anteriorly, and at the same time detaches 

 itself from the epiblast in the region where the blastopore was placed. 

 Where it conies in contact with the wall of the body in front a small 

 epiblastic invagination arises, which meets and opens into the archenteric 

 sack and forms the permanent mouth. 



While these changes have been taking place the shell is formed as a 

 continuous saddle-shaped plate on the dorsal surface. From this plate the 

 two valves are subsequently differentiated. On the dorsal surface they 

 meet with a straight hinge-line. Each valve is at first rounded, but subse- 

 quently becomes triangular with the hinge-line as base. The valves are not 

 quite equi-sided, but the anterior side is less convex than the posterior. At 

 a later period a beak-shaped organ is formed at the apex of each valve in the 

 same manner as the remainder of the shell. This organ is placed at about 

 a right angle with the main portion of the valve. It is pointed at its ex- 



1 In this description I follow Rabl's nomenclature. According to his statements 

 the ventral part of the body is the original animal pole the dorsal the lower pole ; 

 the anterior end the mesoblastic side of the opening of invagination. 



