488 WATASE. [Vol. VI. 



a very early stage of its development, while the female cell 

 is sessile and non-migratory up to the last stage of its ovarian 

 existence. The ovum lies close to the wall of the alveolus 

 and is surrounded with the follicular cells, while the sperm- 

 cell drops into the lumen of the alveolus and there completes 

 its metamorphosis. This difference of conditions, in which 

 the male and female sexual cells respectively develop in the 

 same germ-gland, has been already pointed out by Platner ^ 

 in Afion, and I have witnessed the same fact in a number of 

 other organisms. 



There is still another important feature in which the phenom- 

 ena of sex-differentiation approximate to those of irritability. 

 The most vital peculiarity of irritable organs, as Sachs says, lies 

 less in the fact that their parts can be set in motion in virtue 

 of the unstable equilibrium than in the fact that they subse- 

 quently again resume their irritable condition — their unstable 

 equilibrium. 



Exactly the same phenomenon takes place in connection with 

 sexual differentiation. It is important to bear in mind at the 

 outset certain superficial distinctions that exist between the 

 phenomena of irritability as exhibited in the grozving irrita- 

 ble organism and in the inatitre irritable organ of the adult 

 organism. While the same irritable state of protoplasm returns 

 again and again in an organ after proper restitution of its material, 

 the case is somewhat different in an irritable organism during 

 its growing period, for the simple reason that a greater part 

 of the material which constitutes such an organism has been 

 made irreversible by its growth. As a consequence, the whole 

 mass of the growing organism does not reassume the irritable 

 state in the way that an irritable organ does. But the return 

 of the whole organism to a highly irritable state is attained in 

 quite another way ; namely, by the formation of a unicellular 

 embryo, such as the fertilized ovum in the sexual, and the 

 parthenogenetic ovum in the non-sexual organism. 



To return to the consideration of irritability in the matured 

 organ and in the developing organism, we may observe that 

 all of the material which the organism inherits from the par- 



1 Platner, Zur Bildung der Geschlechtsprodukte bei den Pulmonaten, Arch. 

 / Mikr. Anat., Bd. xxvi, 1886. 



