EMBRYOLOGY AND REPRODUCTION. 67 



plex kind. The type of these is always the same, 

 the male cell being an active moving cell (some- 

 what resembling the flagellate infusorise in form), 

 while the female cell, called the egg-cell, or germ- 



£^ 



Fig, la.^Ova of a Medusa, shon'- 

 iner latge nucleus, with nucleolus. 

 (From Olaus and Sedgwick.) 



Fig. 14. — Spermatozoa of 

 Medusa and of Frog. (From 

 Glaus and Sedgwick.) 



cell, or ovum, is a round cell somewhat resembling 

 the encysted cell already described, and distin- 

 guished by a specially large nucleus, with a distinct 

 nucleolus. The nucleus of the unfertilized egg-cell is 

 sometimes spoken of as the pronucleus. The sperm- 

 cells are developed from cells formed by the division 

 of large cells which look like young ova, and are 

 called " mother cells " of the sperm-cells in all but 

 the simplest animals. The germ-cells and sperm - 

 cells are contained in distinct organs called re- 

 spectively the ovary and the testis, and furnished 

 with efferent ducts, the oviduct and vas deferens re- 

 spectively. Many of the invertebrata present a struc- 

 ture comparable with that of most flowers, the male 

 and female organs being both present together ; but 



