8 



INVERTEBRATE ZOOLOGY 



the parent. Ultimately, the bud may separate from the parent 

 and become an independent organism, or, in some instances, 

 it remains attached permanently and successive generations 

 retain bodily connection, thereby producing a colony. 



Buds usually occur on the external surface of the parent indi- 

 vidual, but in some instances groups of cells within the body 

 become surrounded by a membrane and form what are desig- 

 nated as gemmules or internal buds. Typically, these are highly 

 resistant bodies which arc liberated by the disintegration of the 



Fig. 5. — Eudorina elegans Ehrenberg. A, adult colony, X475; B, daughter 

 colony produced by division of one of the cells of ^4, X730; C—E, development of 

 spermatozoa from a mother cell; F, spermatozoa. {From Shull, LaRue, and 

 Ruthven after Wed and Goehcl). 



body of the dead parent and as resting gemmules tide the species 

 over times and conditions which are unsuited for a vegetative 

 period. The gemmules of sponges and statoblasts of the Polyzoa 

 are among the best examples of internal buds or gemmules. In 

 both origin and structure, these are multicellular. 



Sexual Reproduction. — There are relatively few Metazoa which 

 rely upon asexual reproduction exclusively. More frequently a 

 process involving the specialization of cells for reproduction is 

 encountered. Any cell specialized for this purpose is designated 

 as a gamete or germ cell, and reproduction through such an 

 agency is termed sexual reproduction. Even among the Protozoa 

 (Fig. 5) germ cells are encountered. Despite their relative sim- 

 plicity in organization, there are comparatively few protozoans 

 for which a complete developmental history is known. For 

 some of the most commonly known forms, ignorance of anything 



