EMBRIONIC FISSION IN CYCLOSTOMATOUS POLTZOA. 31 



another instance an injured two-cell embryo developed 

 apparently normally up to the end of the blastophere stage, 

 but finally divided into two ; and in another case the injury 

 to the two-cell stage resulted in the formation of a double 

 monster. 



In Crystallodes (Siphouophora) the remarkable amoeboid 

 character of the superficial blastomeres suggested to Haeckel 

 (13) that the embryo, at the end of the second day, could be 

 compared to a colony of Amoebse, in consequence of the great 

 individuality of the separate blastomeres ; and that, if this 

 comparison were correct, an isolated portion of the embryo 

 might be expected to have the power of further development. 

 The experiments made to test this hypothesis were completely 

 satisfactory. Embryos artificially divided at this stage de- 

 veloped into normal individuals of a smaller size than usual. 

 The cut surface became concave, the edges ultimately joining, 

 so that the embryo again became spherical, and then proceeded 

 to develop in its normal manner. 



It is hardly possible to overlook the fact that, in some at 

 least of the above quoted cases, embryonic fission is specially 

 connected with deviation from the normal type of segmenta- 

 tion of the egg. This is most clearly seen in the case of 

 Oceania, where a superficial glance at Metschiiikoflf^s figures is 

 sufiicient to convince one of the extraordinarily abnormal cha- 

 racter of the segmentation. The same fact is, however, to 

 some extent true of Lumbricus trapezoides and of Crys- 

 tallodes, where Kleinenberg and Haeckel respectively call 

 attention to remarkable features in the segmentation. 



The segmentation of the egg of Crisia obviously belongs to 

 an unusual type, and, as has already been pointed out, it finds 

 its closest parallel in Sal pa, an animal which is remarkable 

 for the great extent to which asexual reproduction is carried 

 out. 



Doliolum, whose life-history agrees with that of Salpa in 

 including two remarkably different generations, offers a further 

 analogy to Crisia in the character of its asexual reproduction. 

 The stolon of the asexual generation segments off, according to 



