THE FACTORS OP ORGANIC EVOLUTION. 65 



with such only of these matters as are put into the food- 

 cavity which it lines. We have further to note that in the 

 embryos of Metazoa at all advanced in organization, there 

 arises between these two layers a third the mesoblast. 

 The origin of this is seen in types where the developmental 

 process is not obscured by the presence of a large food- 

 yolk. While the above-described introversion is taking 

 place, and before the inner surfaces of the resulting epiblast 

 and hypoblast have come into contact, cells, or amoeboid 

 units equivalent to them, are budded off from one or both 

 of these inner surfaces, or some part of one or other ; and 

 these form a layer which eventually lies between the other 

 two a layer which, as this mode of formation implies, 

 never has any converse with the surrounding medium and 

 its contents, or with the nutritive bodies taken in from it. 

 The striking facts to which this description is a necessary 

 introduction, may now be stated. From the outer layer, or 

 epiblast, are developed the permanent epidermis and its 

 out-growths, the nervous system, and the organs of sense. 

 From the introverted layer, or hypoblast, are developed 

 the alimentary canal and those parts of its appended 

 organs, liver, pancreas, &c., which are concerned in deliver 

 ing their secretions into the alimentary canal, as well as the 

 linings of those ramifying tubes in the lungs which convey 

 air to the places where gaseous exchange is effected. And 

 from the mesoblast originate the bones, the muscles, the 

 heart and blood-vessels, and the lymphatics, together with 

 such parts of various internal organs as are most remotely 

 concerned with the outer world. Minor qualifications being 

 admitted, there remain the broad general facts, that out of 

 that part of the external layer which remains permanently 

 external, are developed all the structures which carry on 

 intercourse with the medium and its contents, active and 

 passive ; out of the introverted part of this external layer, 

 are developed the structures which carry on intercourse 

 with the quasi-external substances that are taken into the 



