CELL CONSTANCY IN THE GENUS EORHYNCHUS 289 



5. Brief review oj known extent of cell constancy in the animal 



kingdom 



Lest the impression be conveyed that cell constancy is confined 

 to .occasional forms within a small group of animals the following 

 condensed survey is given of the important work upon this topic. 

 Martini has shown that constancy is present in most and probably 

 in all of the organs of a new-born nematode. The muscle cells of 

 Oxyurias are constant in number throughout life. Goldschmidt 

 has demonstrated cell constancy in the nervous system of As- 

 caris. Brandes has found a definite number of brain cells in 

 Gigantorhynchus, but has not pursued his studies to other or- 

 gans of the body, nor has he deduced any general conclusions. 

 Looss found constancy in the oesophagus of several nematodes. 

 Apathy, working on the central nervous system of Hirudinea, 

 emphasized the constancy found there. Hirschfelder found con- 

 stancy in the midgut of Rotifera, while Martini following him has 

 carefully worked out constancy in the entire body of Hydatina 

 senta, so that he can recognize nine hundred and fifty nine indi- 

 vidual cells, each of which he is able to locate in every of individual 

 of that species. Woltereck has shown a constancy in the cells of 

 the larva of Polygordius. Even in the Chordata constancy has 

 been established by Martini in the various organs of Oikopleura, 

 and Fritillaria. Aside from these definite records given as proofs 

 of constancy by these writers, there exist masses of isolated facts 

 such as have been cited in connection with the reviews of the work 

 upon Acanthocephala in this article, but no attempt has been 

 made to catalogue all of these scattered references. Miss Erd- 

 mann ('12) has given a very general survey of the literature upon 

 cell constancy and the problems closely associated with it. 



6. The significance of cell constancy 



Cell constancy has a most interesting relationship to the prob- 

 lems of comparative anatomy and evolution. Evidently it can 

 occur only in those forms which have a determinate cleavage. In 

 what way has this condition been brought about? Were all forms 

 of development originally of this type or has this condition been 



