264 



SPERMATOGENESIS AND OOGENESIS 



Asexual reproduction takes place by certain of the zooids, 

 which are not ciliated, undergoing a process very like the 

 segmentation of the hydroid egg (p. 245), dividing into 2, 4, 

 8, 1 6, &c. cells (A, a, and D 1 D 5 ), and so forming a daughter 



FIG, 66. Diagram illustrating the hypothetical origin of the gastrula 

 from a colony of unicellular zooids. 



1, simple unicellular form, showing nucleus, ectosarc (Ec), and endo- 

 sarc (En). The black specks represent food-particles. 



2, solid colony of unicellular forms : letters as before except E and F, 

 food particles. 



3, hollow colony of unicellular forms, with central digestive cavity 

 containing food-particles (F). 



4, gastrula formed by the division of each zooid (z) into an ecto- 

 clermal (Ec) and an endodermal (En] cell, and by the formation of a 

 mouth (M) placing the digestive cavity in communication with the ex- 

 terior. (After Lankester. ) 



colony which becomes detached and swims freely in the 

 interior of the parent colony (A), by the rupture of which it 

 is finally liberated. In sexual reproduction certain cells 

 enlarge and take on the characters of ovaries (B, ovy, ovy' 



