SECT. 5] 



IN ONTOGENESIS 



815 



White 



alascewiC3 

 Curtis 

 Komori 

 / O Sendju 

 l^ ID Needham 

 • -Weight of yolk (Sendju) 

 A White] Pre'vostS^Morin 

 XT Yolk j 1846 

 This was first observed by 

 W.Proutinl822 

 ^Water absorption by yolk 

 (current yolkwards) 



1927, and which are shown plotted in Fig. 199. The sharp descent 

 of this curve between the 15th and 20th day must no doubt be due 

 to the formation of what Duval called "the avian placenta". As the 

 two ends of the allantois fuse at the sharp end of the egg they enclose 

 in a vascular bag what remains of the egg-white together with a little 

 yolk squeezed out through the incomplete closure of the yolk-sac. 

 Absorption of the contents of 

 the bag is thus greatly facili- 

 tated, so that by the time of 

 hatching the only reserve of 

 food left is the yolk itself As 

 the contents may be said to 

 consist mainly of protein we 

 should expect to find a marked 

 peak on the curve relating ab- 

 sorption-intensity of protein to 

 time, at this point, and as 

 Fig. 250 shows, we do in fact 

 find such a peak. 



It is next important to note 

 that the amniotic fluid is first 

 of all hypertonic, then isotonic, 

 and lastly hypotonic with re- 

 spect to the embryo, which 

 holds on its way steadily 

 throughout development, until ^^' ^^^' 



at the close of incubation it has almost reached the adult level. 

 The embryo would seem to be independent of its surroundings, and 

 to possess osmotically active substances in constantly increasing 

 amount. It is interesting that chick embryo cells in tissue culture 

 require an isotonic medium for long-continued growth (Ebehng) 

 though they can support hypertonic and hypotonic media for some 

 time. Hogue and Willmer found that osmotic pressure affected cell- 

 migration but according to Lambert and Ebeling it has no effect 

 on cell-multiplication. As for the allantoic fluid, the sudden fall of 

 osmotic pressure towards the end of incubation was brought about, 

 Bialascewicz suggested, by the functioning of the embryonic kidneys, 

 excreting into it dilute urine of low osmotic pressure. Possibly the 

 fall is an index of secretory activity of the metanephros. In all stages 



Water ^ 

 absorption 

 / by chick &. 

 jjj>^evaporation 



