DIFFERENTIATION 1 1 7 



unequal growth, first into the primitive organs and then into 

 the functional organs of the body. " Single sections of the 

 tubes originally formed from the layers develop individual 

 forms, which later acquire special functions : these functions 

 are in the most general way subordinate elements of the 

 function of the whole tube, but yet differ from the functions 

 of other sections. Thus the nerve-tube differentiates into 

 sense-organs, brain and spinal cord, the alimentary tube into 

 mouth cavity, oesophagus, stomach, intestine, respiratory appa- 

 ratus, liver, bladder, etc. This specialisation in development 

 is bound up with increased or diminished growth" (p. 155). 

 Rapid growth concentrated at one point brings about an 

 evagination ; in this manner are formed the sense-organs 

 from the nerve-tube, the liver and lungs from the alimentary 

 tube. Or increased growth over a section of a tube causes it 

 to swell out ; in this wise the brain develops from the nerve- 

 tube, the stomach from the alimentary tube. The segmen- 

 tation which soon becomes so marked, particularly in the 

 muscle layer, is also due to a process of morphological 

 differentiation. 



At the same time that the organs of the body are being 

 thus roughly blocked out and moulded from the germ-layers 

 the third process of differentiation is actively going on. " In 

 addition to the differentiation of the layers, there follows 

 later another differentiation in the substance of the layers, 

 whereby cartilage, muscle and nerve separate out, a part 

 also of the mass becoming fluid and entering the blood- 

 stream " (p. 154). Through histological differentiation the 

 texture of the layers and incipient organs becomes, 

 individualised. In its earliest appearance the germ consists 

 of an almost homogeneous mass, containing clear or dark 

 globules suspended in its substance (ii., p. 92). This 

 homogeneity gives place to heterogeneity ; the structureless 

 mass becomes fibrous to form muscles, hardens to form 

 cartilage or bone, becomes liquid to form the bjlood, 

 differentiates in a hundred other ways into absorbing and, 

 secreting tissues, into nerves and ganglia, and so forth. It 

 will be noticed that the concept of histological differentiation 

 is independent of the cell-theory; it signifies that textural 

 differentiation which leads to the formation of tissues in 



