220 L A MELL1BRA NO HI A TA . 



The foot is large, and ciliated anteriorly. Otolithic sacks and peda 

 ganglia are developed in it very early. 



Unio. The ovum of Anodonta and TJnio is enveloped in a vitelline 

 membrane, the surface of which is raised into a projecting trumpet-like 

 tube perforated at its extremity (fig. 12). This structure is the micropyle. 

 The micropyle disappears in Anodonta. piscinalis when the egg is ripe, 

 but in Unio persists during the whole development. The ova are trans- 

 ported, in a manner not certainly made out, into the space between the two 

 limbs of the outer gills of the mother, and there undergo their early de- 

 velopment. The animal or upper pole of the egg is placed at the pole 

 opposite to the micropyle. 



The segmen tation is unequal (vide p. 82) and results in the formation 

 of a blastosphere with a large segmentation cavity. The greater part of 

 the circumference of the egg is formed of small uniform spheres, but the 

 lower (with reference to the segmentation) pole is taken up by a single large 

 cell. The small spheres become the epiblast, and the large cell gives rise 

 to hypoblast and mesoblast 1 . 



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 Kabl), in contact with each 

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

 considerably 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 

 subsequently 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 2 . 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- 



1 The account of the remainder of the development till the larva becomes hatched 

 is taken from Rabl, No. 292. 



2 IB 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 invagiuation. 



