REPRODUCTION AND DEVELOPMENT. 



377 



the power of completing a whole form. Even animals so 

 complex as lobsters have notably the power of reproducing 

 lost limbs. No such power exists in vertebrate animals after 

 birth, with the exception that various reptiles and fishes 

 renew their tails when they have been accidentally lost; sub- 

 stituting, however, calcified chorda dorsalis for the lost 

 vertebrae. But before birth lost parts may be reproduced, 

 even in man, to a surprising extent. Sometimes, from acci- 

 dental causes, a limb of an unborn child suffers amputation 

 by means of strangula- 

 tion by a band of lymph; 

 and in such cases it often 

 occurs that fingers, or a 

 whole hand, sprout out 

 from the stump of an 

 arm, although they fail 

 to reach the full develop- 

 ment. This is particu- 

 larly interesting, as indi- 

 cating the latent presence 

 throughout the body of 

 the reproductive power 

 which is exhibited nor- Fig. 140. HUMAN OVUM within Graafian 

 rnally and in perfection ? esi ?} Q - a > Germinal vesicle and spot; 



b, vitellus or yelk ; c, zona pellucida; 



d, discus proligerus ; e, membrana 



granulosa ; /, vascular wall of ovisac ; 



g, stroma of ovary; h, surface of 



ovary. 



taining hair and teeth shows the presence of a power which 

 is normally altogether latent till the addition to the ovum 

 of an element containing similar qualities locked up within 

 it causes it to culminate in full reproduction. 



202. The ovum or egg, in birds and various other animals, 

 is enlarged by the incorporation with it of a great amount of 

 matter which does not undergo fertilization, useful only as 

 material for the nourishment of the young animal. But the 

 mammalian ovum is a structure microscopically minute, which 

 is only discovered by scientific observation. 



The human ovum is about y^ of an inch in diameter. 

 It consists of a transparent envelope, the zona pellucida, 



by the reproductive 

 organs alone. So, also, 

 the occasional occurrence 

 of ovarian tumours con- 



