REPORT ON THE DEEP-SEA MEDUSAE. 



xxvu 



subepithelial nervous plexus. The latter, however, want the complete histological 

 independence and the entire separation from the mother-epithelia, already attained by 

 the former. On the whole we find autonomic mesodermal formations chiefly in the 

 higher and larger Acraspedae, in which both the volume of the body and the organological 

 separation have reached a very high grade, whilst they remain at a much lower stage in 

 the smaller Craspedotaa, which are much lower in this respect. If then isolated organs 

 are found on definite parts of the body, in which the different forms of tissue of the 

 animal's body have attained the same high and independent formation, as in the higher 

 animals, there is nothing to prevent us terming these secreted layers of tissue true 

 " secondary germinal layers" (even though these are only developed locally). The two 

 middle plates, the ectodermal muscular plate, and the endodermal connective plate, may 

 be classed together as mesoderm according to the following diagram. 



Diblastic Theory. 



Tetroblastic Theory. 



Triblastic Theory. 



Primitive germinal 

 layer, " Blasto- ■ 

 derma." 



I. Primary dermal layer, 

 " Ectoderma," s.a. ■ 

 ("Ectoblastus"). 



II. Primary intestinal 

 layer, "Endoderma," - 

 s.a. ("Endoblastus"). 



1. Secondary dermal plate 



("Chrotoderma"). } 



2. Muscular plate ("Myo- ( 



derma "). 



3. Connective plate ("Col- 

 loderma "). 



4. Secondary intestinal ] 

 plate (" Gastro V 



derma "). j 



External germinal layer, 

 " Ectoderma," s.st. 



Middle germinal layer, 

 " Mesoderma." 



Inner germinal layer, 

 " Endoderma," s.st. 



§ 43. Differentiation and teleosis of the tissues. The great and general interest pre- 

 sented by the histological structure of the body of the Medusae does not only lie in the fact 

 that we can distinguish the origin of the four secondary germinal layers from the two 

 primary, and especially the derivation of the mesoderm from the two primary germinal 

 layers, more certainly and clearly in them than in the higher Metozoa, but also that in them 

 we can more clearly recognise the mechanical causes of these fundamental processes. 

 These mechanical causes, on the one hand, are the physiological division of labour of the 

 cells and the differentiation of the tissues proceeding from it, and, on the other hand, the 

 physiological perfection of the cells and the progressive development or teleosis of the 

 tissues resulting from it. If these processes of development continue to be carried on now 

 by inheritance in the ontogenesis of the Medusae, the result has been originally brought 

 about in their phylogenesis according to the laws of the theory of selection. 



§ 44. Primary and secondary tissues. The importance of the Medusas for general his- 

 tology lies chiefly in this, that within this class a long series of histological differentiations 



