228 



From what has been said a »nephridium« must be regarded as a 

 compound organ consisting of 1. An egestive aperture 

 which is mesodermal in origin belonging to the coelom 

 andegestive in function, the nephrostome, and 2. the true 

 ectodermal excretory cells, lining a more or less compli- 

 cated tube leading from the nephrostome to the exterior. 



This is borne out by ontogeny *56 {^ go far as the nephridium 

 appears to arise normally, partly, the nephrostome, from the mesoblast, 

 and partly, the tube, from the epiblast. 



The history of the two series of organs can be traced from the dif- 

 fuse egestion and diffuse ectodermal excretion of the sponges (and partly 

 the Echinoderma) to the eventual extinction of the monocytic egestive 

 organs and a perfection of the excretory organs till the Vertebrate 

 kidney shews its nephrostomes only in ontogeny in the highest forms, 

 whereas the excretory ducts and their connection with the vascular 

 system are elaborated to a high degree of perfection (Figs. 5 to 10). 



The Relationship to Ontogeny. 



Note I. Reasons have been given for regarding the modifications of 

 the alimentary processes to be the direct originators of other sets of organs, 

 the instances of skeletal and pigmentary organs being taken as typical. 



If in phylogeny the various organs arise from and are intimately con- 

 nected with, the alimentary processes, then in ontogeny the same will result. 

 The first signs of diö"erentiation will appear in connection with the susten- 

 tative function, and mechanical ingestive processes will lead the way. 



Thus ingestive cells wandering in from the monoblastic single layer 

 will give rise to the inner layer, and the diploplastic form. From this inner 

 layer again the ingestive cells give rise to the mesoblast or third layer, and 

 again in the triploblastic type, — by further in-wandering from the mesoblast 

 the skeletal, connective tissue, sexual and egestive organs arise. If all the 

 organs of a type were monocytic throughout life, we might expect this form 

 of development to prevail, but treading as it were upon the heels of the 

 monocytic organs are the polycytic. Thus the polycytic digestive cavity is 

 formed immediately and consequently upon the monocytic ingestion and 

 again, upon this succeeds the polycytic ingestive area or stomodaeum. In a 

 young form in which the monocytic ingestion and digestion can be dispensed 

 with, the monocytic mechanical inwandering is hastened out of existence 

 and a complete and direct formation of the polycytic organ (archenteric 

 cavity) takes place. 



Thus the complete invagination of a tissue of cells represents the for- 

 mation of the higher type (polycytic) of organ. 



It follows that the highest type of Metazoan ontogeny is represented 

 by such a form as Amphioxus in which nearly the whole development pro- 

 ceeds upon the principle of tissue invagination, the lowest type being a form 

 in which the organs are difi'erentiated by successive migrating of single cells. 



66 E. B. Wilson, Journ. of Morphology. 1887. — Ed. Meyer, Mitth. Zool. 

 Stat. Neapel. VII. 1887. 



