SECT. 5] 



IN ONTOGENESIS 



13 



There would thus seem to be a gradually increasing concentration 

 of osmotically active substances in the silkworm egg as it develops. 

 Polimanti did not make any remark on it, but some possibility evi- 

 dently exists that the fundamental mechanisms here are like those 

 we have already seen to hold in the case of other arthropod embryos. 

 It must be remembered, however, that the water-content of the 

 silkworm egg is not constant, but decreases by evaporation from the 

 time of laying onwards. 



The investigations of the osmotic pressure of the constituents of 

 the bird's egg before and during its development have been few in 

 number, and not very complete. The first study was that of Atkins, 

 who found that the osmotic pressure of the blood of the adult hen 

 was two atmospheres greater than that of the fresh egg (mixed white 

 and yolk). 



AH 



During incubation the osmotic pressure of the white and yolk mixed 

 rose to about that of blood, a phenomenon at first sight Hke that seen 

 in the amphibian and clado- 

 ceran egg, but probably part- 

 ly due to the loss of water by 

 evaporation, and partly to the 

 inorganic salts entering the 

 egg from the shell. Bialas- 

 cewicz later went into the 

 question in detail. His results 

 are shown plotted in Figs. 197 

 and 198. It may be noted 

 at a glance that the osmotic 

 pressure of the embryonic 

 body rises steadily as development goes on, that of the amniotic 

 liquid stands more or less stationary, and that of the allantoic liquid 

 greatly declines. What is the significance of these changes? 



To begin with, the fact that the yolk has a distinctly higher osmotic 

 pressure than the white may at any rate partially explain the passage 

 of water from the white into the yolk, which has been noticed by so 



Fig. 197. 



