ROTATORIA 



267 



does not, it is 

 appear to be 



the fundament of the entoderm, for it is subsequently over- 

 grown by the other cells. The small blastomeres, which now 

 divide repeatedly, are, however, to be considered as ectoderm 

 and mesoderm. 



Particularly striking is the statement that the mesoderm (in the form 

 of three dark, granular cells) arises by division of the small blastomeres 

 that were first to appear, and that it still remains united to the ectoderm, 

 whereas even after its differentiation ectodermal elements continue to be 

 separated off from the large blastomeres. According to 0. Zachiarias, 

 however, the mesoderm is supplied directly by the large blastomercy 

 which on the whole corresponds more to the ordinary mode of formation 

 of the mesoderm, but 

 true, 

 well 

 established in the case 

 under consideration. 

 The conditions of for- 

 mation of the meso- 

 derm hitherto known 

 do not allow a com- 

 parison with the An- 

 nelida, as one would 

 perhaps expect from 

 the relationships of 

 the Rotatoria to these 

 forms. 



The three meso- 

 derm cells lie at the 

 subsequently dorsal 

 side of the embryo 

 (Fig. 115 C). With 

 the progressive 

 division of the 

 ectoderm cells and 

 the commencing 

 circumcrescence 

 of the large blasto- 

 mere by these, the mesoderm cells are pushed farther forward 

 (Fig. 115 D). Meanwhile their number has doubled. Even 

 before the formation of the epibolic gastrula is completed 

 the enclosed entoderm cell has divided. As the result of 



K. H. E. S 



Fig. 115.—^ to F, stages of development of Eosphora 

 digi'tafa (after Tessin). A to C, cleavage stages; D, 

 epibolic gastrula. The large blastomeres are already 

 entirely oTergrown ; tbe mesoderm cells lie at the 

 blastopore. E, the mesoderm cells have moved in» 

 ward ; an invagination of the ectoderm follows it ; 

 the entoderm cells have multiplied. F, embryo, on 

 which the head-, tail-, and lateral lobes can be 

 recognized, iil, blastopore; Ec, ectoderm ;£», ento- 

 derm ; Mes, mesoderm. 



