SECT. 6] 



OF THE EMBRYO 



873 



Erithracus rubecula 



cently extended it to the eggs of many other birds, mostly wild species. 

 Their data are fragmentary, and not suitable for compression, but 

 in all cases they found that the 

 larger the embryo, the less was its 

 water-content. They also found a 

 falling weight of the whole egg 

 during development, four ex- 

 amples of which are given in 

 Fig. 222. The amount of weight 

 lost by eggs of various birds dif- 

 fered, but never exceeds 30 per 

 cent of the original weight. 

 Table loi gives these differences 

 concisely. For a given bird, such 

 as the hen, the loss is so constant 

 that Zunz suggested frequent 



Fig. 222. 



weighing as a guide to regulation of normal development. It is 

 interesting to recall that other terrestrial eggs, e.g. the silkworm 

 (Luciani & Piutti) lose water as they develop. 



Table 10 1. Loss of weight by fertile eggs during incubation. 



Species 



Hen ( Callus domesticus) 



Hen( „ „ ) 



Hawk {Buteo buteo) ... 



Falcon {Cerchneis tinnunculus) 



Pheasant [Phasianus colchicus) 



Nightingale {Turdus philomelos) 



Yellow-hammer {Emberiza citrinella sylvestris) 



Chaffinch {Fringilla coelebs) 



Linnet {Acanthis cannabina) 



Robin {Erithracus rubecula) 



Sparrow {Prunella modularis) ... 



Warbler {Sylvia communis) ... ... 



Starling 



N.B. This progressive water loss is one of the main difficulties in the way of successful 

 in vitro incubation of the avian embryo, a technical problem which has not yet been 

 solved; see Loisel (2 or 3 days), Vogelaar & Boogert (6 days), Fere; McWhorter & 

 Whipple and S. Paton. 



Iljin and Alcacid working with different incubators, some using 

 wet air and others dry, observed a greater loss of weight in the latter, 

 but the most complete examination of evaporation-rate is that of 

 Murray. His immediate aim was to find the optimum conditions for 



56-2 - 



