502 THE SOCIAL ORGANISM. 



T!JC doctrine of the progressive division of labour, tc 

 which we are here introduced, is familiar to all readers. 

 And further, the analogy between the economical division 

 of labour and the &quot; physiological division of labour,&quot; is so 

 striking, as long since to have drawn the attention of sci 

 entific naturalists : so striking, indeed, that the expression 

 &quot; physiological division of labour,&quot; lias been suggested by 

 it. It is not needful, therefore, that we should treat this 

 part of our subject in great detail. We shall content our 

 selves with noting a few general and significant facts, not 

 manifest on a first inspection. 



Throughout the whole animal kingdom, from the Cce- 

 k/tterata upwards, the first stage of evolution is the same. 

 Equally in the germ of a polype and in the human ovum, 

 the aggregated mass of cells out of which the creature is 

 to arise, gives origin to a peripheral layer of cells, slightly 

 dirtl-ring from the rest which they include ; and this iaycr 

 subsequently divides into two the inner, lying in contact 

 with the included yelk, being called the mucous layer, and 

 the outer, exposed to surrounding agencies, being called 

 the serous layer : or, in the terms used by 1 rof. IIuxK y, in 

 describing the development of the Jfydrozoa the emto- 

 denn and ectoderm. This primary division marks out a 

 fundamental contrast of parts in the future organism. 

 From the mucous layer, or endoderm, is developed the 

 apparatus of nutrition ; while from the serous layer, (&amp;gt;r ec 

 toderm, is developed the apparatus of external action. 

 Out of the one arise the organs by which food is prepared 

 and absorbed, oxygen imbibed, and blood purified; while 

 out of the other arise the nervous, muscular, and osseous 

 systems, by whose combined actions the movements uf the 

 body as a whole arc effected. Though this is not a rigor 

 ously-correct distinction, seeing that some organs involve 

 both of these primitive membranes, yet high authorities 

 agree in stating it as a brojid general distinction. 



