SECT. 6] OF THE EMBRYO 887 



Wetzel's comparative work introduces further complications. He 

 analysed the eggs of a number of different kinds of animals, and then 

 adopted the unsatisfactory expedient of comparing the results with 

 figures for adults of the same kind, taken from other investigations. 

 Thus he drew up the following table : 



Table 104. 



Water-content % 



But we cannot conclude from this table that the echinoderms get 

 drier as they develop or that the Crustacea and elasmobranchs get 

 wetter, for we learn nothing from it about what is the real centre 

 of interest, the embryonic body itself. According to Ephrussi & 

 Rapkine the sea-urchin's egg absorbs water from the sea. 



Wet weight Dry weight 

 Change: unfertilised egg as unity (- J-;; ;;; t^iS +9;o 



Just as there is still much uncertainty about the behaviour of water 

 in the entire embryonic organism, so the data we have for its separate 

 tissues are rather complicated, if not contradictory. For muscle 

 tissue, Jacubovitsch stated in 1893 that the water-content decreased 

 with the age of the embryo; his figures are shown plotted in Fig, 231 

 together with those of Mendel & Leavenworth on the brain and liver 

 of the pig. Bischov, again, had found the figures: 



Then Faure-Fremiet & Dragoiu, in their work on the embryonic 

 lung in the sheep, obtained a curve which fell from the 7th to the 

 14th week, after which the determinations became impossible owing 

 to the swallowed amniotic liquid. As regards nervous tissue, Glaser's 

 results on the brains and spinal cords of Amblystoma indicated a 

 constancy in water-content; thus the just-hatched Amblystoma brain 



