ONTOGENY 



off by the other, forming new individuals smaller than the 

 parent form, but otherwise similar to it (Fig. 4). This fission 

 in the Metazoa is not really comparable with the similarly 

 named process in the Protozoa; it represents a special acquire- 

 ment and is usually, though not always, associated with other 

 more complex modes of reproduction. Normal fission is 

 known to occur in many genera among the Coelenterates, and 

 less frequently among the Porifera, Platyhelminthes, Annulata, 

 Bryozoa, and Echinoderms. 



It is obvious that in reproduction by fis- 

 sion the individuality of the parent organism 

 is lost in the act of giving rise to the new 

 individuals, although none of the substance ] 

 of the original organism perishes in the act. 



FIG. 4. Fission in Metazoa. A, B. Two stages in the transverse fission of 

 the Actinian, Gonactinia prolifera. From Korschelt and Heider, after Blochmann 

 and Hilger. C. Successive transverse fissions in the Platyhelminth, Micro- 

 stomum lineare. After L. von Graff. /, //, III, mark the levels of the successive 

 fissions; a fourth fission also is indicated, p, p, pharynges. 



In the fission of Metazoa usually many of the structures of the 

 parent are simply transferred to the new organisms with com- 

 paratively little differentiation of parts anew, out of a visibly 

 undifferentiated condition. In the Protozoa this may or 

 may not be the case. In some of the highly organized 

 Ciliata most of the structural differentiation disappears just 

 previous to fission, after which each daughter cell differ- 

 entiates a typical form and structure anew (Fig. 5). In 

 other Protozoa there is a considerable transference of char- 

 acteristics accompanied by a lesser amount of regeneration or 



