REPRODUCTION IN THE GASTR^ADS. 6g 



takes place directly through the stomach-cavity. A true 

 palingenetic gastrula (Fig. 174) develops, just as in the 

 Monoxenia (Fig. 171), from the fertilized egg. This swims 

 a^bout for a time in the sea, then attaches itself, and in this 

 state resembles a simple young-form, which occurs in the 

 course of the evolution of many other Plant- Animals, and 

 which is called the ascula (Figs. 182, 183). In consequence 

 of the absorption of foreign bodies by the exoderm, it 

 becomes the Halii^hysema. 



When we consider that there is no other important 

 difference between the free-swimming gastrula and this 

 attached, simplest Plant-animal, we are fairly justified in 

 stating that in the simplest form of Gastrsea sexual repro- 

 duction must have taken place in the same way. In the 

 Gastrseads, just as in Plant-animals, both kinds of sexual 

 cells — egg-cells and sperm-cells — must have formed in the 

 same person; the oldest Gastrceads must, therefore, have 

 been hermaphrodite. For Comparative Anatomy shows 

 that hermaphroditism, that is, the union of both kinds of 

 sexual cells in one individual, is the oldest and original con- 

 dition of sexual differentiation ; the separation of the sexes 

 {GonocJiorismus) did not originate till a later period. 



