

Biogen in the Ovarian Egg. 397 



shall therefore designate it by another name, and propose to call 

 it from its physiological significance, Biogen, or the biogen liquid. 

 Now if the yolk is really the product of the biogen liquid, this 

 explains the absence of a separate covering or membrane around 

 it. Just as in the ovarian egg of the hen or the frog, so in the 

 ovarian egg of the Ascidia and of all Mollusks, there is but 

 one membrane. The second, or shell membrane, is not acquired 

 in these higher animals before the passage of the egg through the 

 oviduct. But as nothing of the kind takes place in Mollusks, it is 

 natural that their eggs, even after their expulsion from the ovary, 

 should have only a single membrane. It is only because the 

 transformation of the immature into the mature egg was over- 

 looked, that the error has been committed of considering the 

 outer membrane of fig. 9, as different from that of figs. 6 and 7. 

 In fact however, there is no difference between them except that 

 in fig. 9, the yolk-substance has retreated from the edges and 

 been concentrated into a compact ball. The biogen fluid itself 

 undergoes essential modifications in the course of the develop- 

 ment. At first it is tolerably compact, and in many ovarian egus 

 even gelatinous. By degrees it becomes thinner and at last quite 

 fluid, so that it seems to afford no important hindrance to the re- 

 volving embryo. 



How far this mode of formation of the yolk out of the biogen 



liquid is applicable to the eggs of vertebrate animals, must be 



learned from future observations. Something of the kind is 



certainly to be seen in the eggs of Mammalia. According to 



! Bischoff's investigations, the yolk in the mature egg of the rab- 





bit fills the whole zone. Some days later, however, after it has 

 I left the ovary, there appears between the zone and the yolk a 





clear space, filled with a transparent fluid.* This fluid, to which 

 Bischotf particularly calls attention, has, we believe, never been 



J satisfactorily explained. For it is hardly to be supposed that it 



is water, received by means of endosmosis, as has been sup- 

 posed m the case of the eggs of fishes; for, from whence should 

 the water come? But on the supposition that the yolk condenses, 

 the matter is quite simple. The clear fluid is in that case noth- 

 ing else than a part of the biogen liquid, which has been separa- 

 ted by the condensation of the yolk. 



If this explanation is well founded, there must be a time when 

 the mammalian egg also is transparent, that is, before the separa- 

 tion of the yolk-granules or vesicles has begun. And indeed I 

 should not be astonished if the vesicles with inner cells, which 

 Bischoff figures and describes as probably immature eggs, should 

 prove to be the analogue of our Ascidian eggs, as represented in 



figs. 1-5. 



* Bi^cLofl^ Daa KaaiueLeii-Ei. Tab. I 



