CLEAVAGE 



253 



12212 

 Ib 112212 

 10112212 

 1^112212 



4d 



primary ganglion 

 cells. 



mesoblast. 



4d 5 



4d* alimentary canal (endoderm). 



4d 211 probably endo- 

 derm. 



4d 212 mesoderm (right 

 "mesoblast band"). 



4d 221 probably endo- 

 derm. 



4d 222 mesoderm (left 

 " mesoblast band"). 



The remaining cells form covering ectoderm. 

 Ectoderm of first quartet anterior and dorsal, including eyes. 

 Ectoderm of second quartet posterior and ventral, including pharyngeal. 

 Mesoderm of second quartet blastoporal (pharyngeal). 



"The nuclei of the small macromeres [4A, 4B, 4C, 4D] show 

 evidences of degeneration. These do not divide as long as 



they can be followed and it seems probable that 



they degenerate without giving rise to any morphological 

 structure." 



This cell lineage of Planocera is summarized incompletely in 

 the accompanying table. 



It is interesting to compare with this lineage of Planocera 

 that of Ascaris, described by Zur Strassen, which is somewhat 

 less regular. This is particularly interesting as it shows 

 clearly the history of the germ cells, which become wholly 

 separate from somatic cells in the sixteen-cell stage. Cleavage 

 of Ascaris (Fig. 121) is bilateral but more or less irregular/ 

 particularly in its rhythms, so that without attempting to 

 apply the ordinary terminology completely we may summarize 

 the early cell history in the table accompanying (p. 255). 



The cell lineage of a considerable number of organisms has 

 been definitely traced, often in much greater detail than we 

 have indicated. The histories best known are found among 

 the Platyhelminthes, Nemathelminthes, Nemertinea, Annulata, 

 Trochelminthes, Mollusca, and Tunicata. Of course the eggs of 

 many classes and phyla show no such regularity, for as we have 



