Vertebrate Embryos and Medusae 43 



contrasted in the strongest way with the group Ccelom- 

 ata, in which are placed all the higher animals, from the 

 simplest worm up to man ; animals in which, in addi- 

 tion to the two foundation-membranes of the Coelen- 

 terata, there is a third foundation-membrane, and in 

 which, in addition to the simple stomach cavit}- with 

 its offshoots, there is a true bod5'-cavity or coelome, and 

 usually a set of spaces and channels containing a blood- 

 fluid. The older method of naming groups of animals 

 after some obvious superficial character lingered on for 

 some years in text-books and treatises, but in this 

 memoir the young ship-surgeon had replaced it by the 

 modern scientific method of grouping animals together 

 onl}' because of real identity of structure. 



There is yet left to be noticed perhaps the most won- 

 derful of all the ideas in this first memoir by Huxley. 

 In the course of describing the two foundation mem- 

 branes of the Medusae he remarks: 



"It is curious to remark, that throughout, the outer and 

 inner membranes appear to bear the same physiological rela- 

 tion to one another as do the serous and mucous layers of the 

 germ : the outer becoming developed into the muscular system, 

 and giving rise to the organs of offence and defence : the inner 

 on the other hand appearing to be more closely subservient to 

 the purposes of nutrition and generation." 



In the whole range of science it would be difficult to 

 select an utterance more prophetic of future knowledge 

 than these few words. Huxley had been reading the 

 investigations of Von Baer into the early development 

 of back-boned animals. He had learned from them the 

 great generalisation, that the younger stages of these 

 animals resemble one another more closely than the 

 adult stages, and that in an early stage in the develop- 

 ment of all these animals the beginning of the embryo 



