KLEINENBERG ON DEVELOPMENT OP LOPADORIIYNCMUS. 515 



stitution, having done which they may either disappear (gill- 

 slits of Vertebrates as such), or remain as obviously rudimentary 

 structures. 



The above considerations must lead to an entire recodifica- 

 tion of the conventional assumption of a mesoblast. There is 

 nothing in morphology about which there is so much disagree- 

 ment as the homology of the mesoblast and the cavities included 

 in it. The most diverse origins have been assigned to this 

 supposed germ-layer ; it arises from the outer germ-layer alone, 

 from the inner germ-layer alone, from both together, from 

 neither, from two special blastomeres, from every part of the 

 living blastoderm (Sarasin), not from the blastoderm at all, 

 but from the yolk (His). With so many contradictions before 

 one, it is at least a step in advance when we can strike out on 

 a fresh line, and begin a new series of observations on the 

 assumption that there is no mesoblast. Whether Kleinenberg's 

 views will stand the test of further proof is of course an open 

 question, but it cannot be doubted that investigations under- 

 taken from his point of view must be fertile of results. To 

 trace back organs through the intermediary organs which gave 

 them origin, and to establish homologies between the latter 

 and the permanent organs of lower forms, is a task which 

 offers many advantages to the researcher, and is likely to 

 vield much surer and more satisfactory results than the 

 attempt to refer an organ merely to one of the three primary 

 germ-layers. 



It must not be forgotten that the homologies of the meso- 

 blast and coelom are two of the most debateable questions in 

 morphology. But, as Kleinenberg says, a hole is a hole the 

 world over, — if we can establish the homologies of the walls 

 bounding a cavity, there is no need to quarrel about the empty 

 space itself. Already one can see the possibilities of satisfac- 

 tory explanations, through the theory of substitution, of the 

 coelomic cavities of Arthropods, the greater part of which is 

 shown by Sedgwick to be turgid blood-vascular space, and the 

 various stages of coelosis in the Hirudinea. Kleinenberg states 

 that the coelom of Lopadorhyuchus is what would be conven- 



