ORTHONECTID^ AND DICYEMIDA] 



211 



By increasing in length the embryo becomes vermiform, 

 whence its name (Fig. 99 B and C). These embryos are not 

 essentially different from the 

 adult animal, whose shape is soon 

 fully assumed by the accomplish- 

 ment of the slight differentiations 

 in the outer layer of the body and 

 in the head region, and by the 

 elongation becoming more pro- 

 nounced (Fig. 99 C and D). Then 

 the formation of new germs in 

 the axial cell begins very early, 

 in fact while the embryo still 

 remains within the parent. The 

 processes described apply there- 

 fore to embryos which are still 

 found within the parent (Fig. 99 

 Ato D). When they have arrived 

 at maturity, they break through 

 the outer layer of the parent, but 

 remain in the venous appendages 

 of the Cephalopods, where they 

 still grow considerably and pro- 

 duce other embryos. 



Structure and Develop- 

 ment of the Infusoriform 

 Embryos. — The infusoriform 

 embryos differ widely from the 

 vermiform in shape. Of a shorter, 

 more compressed form, they also present numerous internal 

 differentiations (Fig. 100 DtoF). In swimming, the broader 

 end of the embryo is directed forwards. Whereas the an- 

 terior end is naked, the rest of the body is ciliated (Fig. 100 

 C and D). The entire embryo is constructed on the bilateral 

 plan, for two lateral parts as well as a dorsal and ventral 

 side can be distinguished. Anteriorly and more dorsally lie 

 two highly refractive bodies (Fig. 100 D,r), somewhat behind 

 them, and lying more ventrally, the organ called by van 

 Beneden the "urn." This peculiar organ, the function of 



Fig. 99.-4 to D, stages in tlie 

 development of the veimiform 

 embryos of Dicyema ; A, of Vi- 

 ctjemennea eledoiies (after Whii- 

 man); B to D, of Dicyema typu^ 

 (after B. van Beneden). Ax, a.xial 

 cell ; K, nucleus of the axial cell ; 

 Ks, germ cells. 



