112 Dr. M. Ussow on a new 



cylindrical, hollow, and csecal tube (PI. IV. fig. 3), 15-17 

 millim. long and 1^-2 millira. in thickness, which was covered 

 on the surface with primary buds (a). 



The walls of this tube consist of a single layer of ectoderm 

 and a single layer of endoderm. Even in the youngest deve- 

 lopmental stages we observe also over the whole body be- 

 tween tliese two layers, but more closely applied to the ectoderm, 

 some elongated fusiform cells ; they form a unitary muscular 

 layer, which in further growth becomes sharply marked — the 

 mesoderm. 



Simultaneously with the advancing formation of a muscular 

 lamina, the cord-like body, alternately contracting and extend- 

 ing itself in the direction of its length, twists itself into a 

 spiral (of three or four turns). This surrounds a central yolk- 

 mass, while a few yolk- globules pass to the periphery of the 

 ovum through the turns of the spiral, and take their place 

 between the chorion and the primary buds. The spiral turns 

 follow the long axis of the spheroidal ovum of the Sterlet. 

 The primary buds upon the body of the parasite, which has 

 neither moutli nor anus, have at first the appearance of not 

 "very strongly marked rounded swellings, which, by the con- 

 striction of their base at the body of their bearer, gradually 

 become distinct and acquire a pyriform shape. Tlie axial 

 cavity of the general bearer is then continued into the spacious 

 cavity of each bud, the delicate walls of which form a con- 

 tinuation of the three cell-layers, the ecto-, meso-, and endo- 

 derm. 



Soon after the pushing out of the buds there appears upon 

 each of them a slight furrow, which gradually deepens and 

 effects the division of each of the primary buds into two like- 

 wise pyriform bodies, the secondary huds (fig. 4). These, in 

 the sequel, become developed into free-living forms (mothers). 

 Each eight secondary, representing four primary buds, takes 

 part in a complete spiral turn (4x8 = 32) , at the same time 

 they bend at an angle to one another during the twisting of 

 the general support [stolo'^), and place themselves all on one 

 side of the stolo. The side of the parasite which is free from 

 buds is turned towards the central yolk-nucleus, while the 

 upper parts of all the thirty-two buds are turned towards tlie 

 peripheral yolk, or the chorion. 



Upon the upper part of each secondary bud there is also a 

 slight furrow, which, however, does not penetrate deeply, but 

 only indicatively divides the cavity of the bud into a right and 



* I sliall make use of this designation, altliougli, as will be seen here- 

 after, this part does not represent morphologically a stolo sens, strict. ; I 

 choose the expression merely for convenience. 



