THE OVARY AND ITS IMPORTANCE 



55 



individuals may arise by budding from a stolon-like base of the parent (fig. 

 27). This process often is called gemmation, the formation of a new indi- 

 vidual by a protrusion of a mass of cells from the parental body followed by 

 its partial or complete separation. It is a prominent method of reproduction 

 among the lower Metazoa, particularly the coelenterates and sponges. Never- 

 theless, all animal species among the Metazoa ultimately utilize an egg as 

 the primordium from which the new individual arises. Sexual reproduction, 

 generally associated with the fertilization of an egg by a sperm element, appears 

 to be a needful biological process. 



True as the general statement made by Harvey may be, it is not clear what 

 is meant by the word ovum or egg. We know certain of its characteristics, 

 but, for the most part, it must be accepted as an accomplished fact enshrouded 

 in mystery. To Harvey the egg was an indefinite, unorganized association of 

 substance plus a "primordial generative principle" (see Cole, F. J., '30, p. 

 1 40 ) . Other minds have conceived of other meanings. Nevertheless, descriptive 

 and experimental embryology has forced the conclusion that the egg, during 

 its development within the ovary, experiences a profound process of differ- 

 entiation, resulting in the formation of an invisible organization. Although 



STOMACH 



ESOPHAGE AL 



1 E N T 18 2 



A R Y N X 2 

 H A GUS 2 



—STOMACH 



Fig. 27. Forms of asexual reproduction in the subphylum Urochordata of the phylum 

 Chordata. (From MacBride: Textbook of Embryology, Vol. 1, London. Macmillan.) 

 (A) Budding from 'stolon of Perophora listen, from MacBride after Hjort. (B), (C) 

 Two stages of budding in an ascidian, from MacBride after Pizon. 



