THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 539 



way by embracing these organs in a chapter by themselves and 

 discussing them apart from the organs of the inner, middle, and 

 outer germ-layers. 



It is the original province of the intermediate layer to form a 

 packing and sustentative substance between the epithelial layers, a 

 fact which stands out with the greatest distinctness particularly in 

 the lower groups, as for example in the Ccelenterates. It is there- 

 fore closely dependent upon the epithelial layers in the matter of its 

 distribution. When the germ-layers are raised up into folds, it 

 penetrates between the layers of the fold as a sustentative lamella ; 

 when the germ-layers are folded inwards, it receives the parts that 

 are being differentiated as for example in the Vertebrates, the neural 

 tube, the masses of the transversely striped muscles, the secretory 

 parenchyma of glands, the optic cups, and the auditory vesicles 

 and provides them with a special envelopment that adjusts itself 

 to them (the membranes of the brain, the perimysium, and the 

 connective-tissue substance of the glands). In consequence of this 

 the intermediate layer, in the same proportion as the germ-layers 

 become more fully organised, becomes itself converted into an extra- 

 ordinarily complicated framework, and resolved into the most diver- 

 gent organs, by the formation of evaginations and invaginations 

 and the constricting off of parts. 



The form of the intermediate layer thus produced is of a second- 

 ary nature, for it is dependent upon the metamorphosis of the germ- 

 layers, with which it is most intimately connected. But in addition, 

 the intermediate layer, owing to its own great power of metamor- 

 phosis, acquires in all higher organisms, particularly in the Verte- 

 brates, an intricate structure, especially in the way of liistological 

 differentiation or metcvnwrphosis. In this way it gives rise to a 

 long series of various organs the cartilaginous and bony skeletal 

 parts, the fasciae, aponeuroses, and tendons, the blood-vessels and 

 lymphatic glands, etc. 



It is therefore fitting to enter here some\vhat more particularry 

 upon a discussion of the principle of histoloyical differentiation, and 

 especially to inquire in what manner it is concerned in the origin of 

 organs differentiated in the mesenchyme. 



The most primitive and simplest form of mesenchyme is gelatinous 

 tissue. Not only does it predominate in the lower groups of animals, 

 but it is also the first to be developed in all Vertebrates, out of the em- 

 bryonic cells of the intermediate layer, and is here the forerunner and 

 the foundation of all the remaining forms of sustentative substance. 



