580 SUMMARY OF CURRENT RESEARCHES RELATING TO 



undifferentiated cells. The larva has its own nervous system and its 

 own musculature, no parts of cither of which pass over to the adult, but 

 are replaced by a new nervous system and musculature, and arc after- 

 wards aborted. The central nervous system of the adult is formed from 

 a number of separate centres or foundations, the situation of which is in 

 part determined by the pre-existing nerve-centres of the larva, although 

 the latter have no direct share in their formation, but act only as foci, in 

 connection with which fresh groups of nervous elements are differentiated. 

 The muscle-plates do not split into two layers, comparable to the 

 Bomatopleure and splanchnopleure, and the coilom is formed simply as 

 an extension of the space existing between ectoderm and endoderm, the 

 peritoneal walls of which are formed by cells derived from the muscle- 

 plates. All the tissues usually classed as mesoblastic are derived from 

 the ectoderm, and the endoderm appears to form nothing more than the 

 lining of the gut. 



Kleinenberg does not believe tliat a new organ is formed by the 

 gradual growth or change of a pre-existing organ. " In animals of simple 

 organization, tissues and organs are formed to which special functions 

 are appropriate. Their functional activity is, as it were, a disturbing 

 element in the organism ; it induces changes in neighbouring tissues, 

 and gives the signal for new specializations in them. The functional 

 activities of the newly specialized tissues must always bear some relation 

 to the function of the organ which determined their origin, and must 

 either support or modify their action. The newly formed tissues again 

 affect the organism ; their importance increases, and they may in time 

 give rise to fresh tissues. Finally, they may become so important that 

 they outweigh in functional importance the organ to which their origin 

 was due ; they then take its place, and the latter dwindles till finally it 

 may disappear altogether. This process, which is of the greatest 

 importance, is called by Kleinenberg the development of organs by sub- 

 stitution. ... In no case of substitution are the intermediate steps 

 represented by an indifferent germ-layer, but always by a functional 

 and specifically differentiated organ." A striking instance of this is the 

 development of the axial skeleton of the Chordata ; another is the 

 successive development of pro-, meso-, and metanephros. "Rudimentary" 

 organs are thus seen to be intermediary. 



It is an open question whether Kleinenberg's views will stand the test 

 of further proof, but it cannot be doubted that investigations undertaken 

 from his point of view must be fertile in results. It is better to trace 

 back organs through the intermediary organs which gave them origin, 

 and to attempt to establish homologies between the latter and the per- 

 manent organs of lower forms than to attempt to refer an organ merely 

 to one of the three primary germ-layers. 



Experiments on Earthworms. f — Herr W. Kiikenthal describes some 

 interesting observations and experiments which he made on earthworms. 

 After noting the nature of the ^cretion which comes from the body, 

 which includes entire glandular cells from the hypodermis, he describes 

 how he fed the animals with carmine and indigo in order to test whether 

 the above glandular cells were not in certain conditions excretory. The 

 result proved that the granules of carmine were taken up by the cells of 

 the gut which become amoeboid. Lang observed a similar amceboid 



* Biol. Centralbl., viii. (1888) pp. 80-6. 



