PLATYHELMINTHES 



105 



Fig. 80. — Developing egg of Planocem 

 inquilina. Eight - cell stage viewed 

 from animal pole. (After Surface. ) 



The 8-cell stage is succeeded by a 16-cell stage. The micro- 

 meres divide into upper and lower cells by laeotropic spindles, 



and by four laeotropic spindles 



four new micromeres are budded 



off which alternate with the lower 



daughter cells of the first micro- 

 meres. (Fig. 81.) 



The 16-cell stage is succeeded 



by a 32-cell stage. By dexio- 



tropic spindles the macromeres 



bud off a third set of micromeres. 



The second micromeres divide 



into upper and lower cells, whilst 



each daughter of the first set of 



micromeres divides into an upper 



and lower cell. 



The subsequent history of the 



embryo proves that in Planocera, 



as in every Annelid and Mollusc 



that has been examined, the three groups of micromeres and their 



daughters constitute the entire ectoderm, whilst what is left of the 



macromeres, after the separa- 

 tion of these micromeres, 

 gives rise to the endoderm 

 and mesoderm. 



Now if the four macro- 

 meres were precisely equal in 

 size it would be of course im- 

 possible to distinguish them 

 from one another, but in 

 Planocera, as in the vast 

 majority of cases, one is 

 slightly larger than the rest 

 and distinguishes itself by 

 peouharities in its develop- 

 ment after the micromeres 

 have been given off; this 

 macromere is denominated D. 

 In all cases where it is recog- 

 nizable from the first it forms 

 a landmark by means of 



which the cleavage planes of the egg can be . correlated with the 



planes of symmetry of the adult. 



It is thus found that D is situated on what will be the posterior 



side of the embryo in the middle line. The other cells are named 

 A, B, and C, following each other round a circle from left to right, 

 in the same direction as the hands of a clock move when viewed from 

 above. A and C are situated on the left and right sides respectively 



Fig. 81. — Developing egg ot Planocera ingiuUina. 

 Sixteen-cell stage viewed from tie animal pole. 

 (After Surface. ) 



