SECT. 6] OF THE EMBRYO 897 



that phyllopod eggs would withstand 14 years' drying in a desiccator 

 without losing their power of development and Pirie has exposed 

 Artemia eggs to phosphorus pentoxide at less than | mm. pressure for 

 several days after which development proceeded normally. Carpenter 

 noted much the same facts in the case of rotifer eggs. When they are 

 put into water they develop : all the water in the body of the embryo 

 being derived from the environment except that small trace which is 

 bound to the hydrophihc colloids of the ^gg (see Gortner & Newton, 

 and Robinson). Doubtless this hardiness is an adaptation to the 

 seasonal drying up of the pools in which these animals live. 



There is strong reason to believe, therefore, that the eggs of aquatic 

 animals do not contain enough water to make their end products, 

 but have to absorb it from their surroundings. It is possible that 

 some obscure phenomena may receive an explanation on these 

 grounds; thus the eggs of the pike (Esox), according to Kasanski, 

 rotate round and round within their membranes, after only 24 hours 

 cleavage; and this movement, which continues until the muscles are 

 formed, may be an adaptation for ensuring water-intake, by setting 

 up currents within the egg-case. And Amemiya states that the eggs 

 of the fresh-water teleost, Oryzias talipes, show conspicuous undulating 

 movements of the blastoderm from an extremely early stage onwards. 

 Giard, long ago, showed that many aquatic eggs (fresh-water mol- 

 luscs, Hirudinea, marine polychaetes, molluscs and nudibranchs) 

 would develop well if kept in moist trays, not actually immersed in 

 water, but that moisture was essential. Certainly in many cases, 

 as we have seen, the percentage dry weight of aquatic eggs is 

 much higher than that of the corresponding fully formed tissues. 



"If then", said Gray, "it may be assumed that most, if not all, 

 aquatic organisms are dependent on the environment for a supply of 

 water, an interesting problem of phylogeny is opened up. The 

 primitive vertebrate is, with good reason, regarded as the offspring 

 of an aquatic type, but at some stage in the history of the truly 

 terrestrial animals there must have come a time when oviposition 

 occurred on land and not in water. During the earlier stages of their 

 evolution terrestrial vertebrates no doubt laid their eggs in water 

 where possibly the factor of greatest survival value consisted in the 

 newly hatched individual having reached a stage at which it might 

 fend for itself and be of active habit. Now an animal such as the 

 trout necessarily hatches at an early stage since the increasing volume 



