SECT. 6] OF THE EMBRYO 889 



centage had been contributed by each of the tissues. His figures were 

 striking. 



There is thus the possibility that the decreasing water-content of 

 embryos may be simply an index of the decreasing amount of primi- 

 tive connective tissue. From the point of view of paraplasmatisation 

 or degree of diminution of active protoplasm, it would be desirable 

 to test the truth of this view in some way. The original suggestion of 

 connective tissue and lymph as playing this role is due to Schloss- 

 berger. From the experiments of Wiener we know that, in the 

 human foetus, the lymph circulation is active from an early date. 

 Foetal lymph has been analysed by Raske (see Section 23-7). 



6-6. Water-absorption and the Evolution of the Terrestrial 

 Egg 



We may now return to the curve for the rising water-content of 

 the amphibian larva (embryo plus yolk) established by so many 

 workers and plotted in Fig. 230. It has usually been regarded as a 

 fundamental property of this phase of embryonic growth, but Arager 

 contests such a view. Arager removed the jellies from frog neurulae, 

 and put them to develop in a chamber the humidity of which could 

 be controlled. In this way it was possible to produce larvae of very 

 much less water-content than normal, and Arager noted that, in 

 such larvae, the histological differentiation of the tissues was normal, 

 although the morphological relations of the organs might be con- 

 siderably upset. 



But leaving this difficult question, we shall find it advantageous 

 to study the results obtained by Kronfeld & Scheminzki and by 

 Gray on the trout embryo. As has before been mentioned, this can 

 be separated from its yolk at an early period. In general, it resembles 

 the amphibia, for a preliminary period inside the egg-membranes is 

 succeeded by a free-swimming period in which the fish lives on the 

 yolk in its yolk-sac, and this in turn by a period in which ordinary 

 ingestion of food by the mouth is begun if there is anything present 

 to eat. As Fig. 232 shows, the water-content of the trout embryo is 



