THEORIES OF THE CCELOM 21 



interesting work, Die Coelomtheorie (Jena, 1881), definitely denied 

 to this space the nature of coelom. They called it " pseudocoel," 

 and in the same category they placed the body-cavities of the 

 Kotifera, the Polyzoa, and the intercellular spaces of the paren- 

 chyma of Platyhelmia. The remaining groups of animals (exclusive 

 of the Ccelentera of Leuckart) they credited with the possession of 

 a true coelom, which they considered as being always an entero- 

 coel in origin. 



The Hertwigs thus practically accepted my theory of the origin 

 and nature of the true coelom, but rightly refused to include in 

 this category the blood-holding space of the Molluscs. If I proceed 

 to point out where they were mistaken it is in no spirit of 

 reproach, for their work has in this and again in the history of 

 the fertilisation of the egg-cell been of capital importance. It is 

 necessary, as we push our way through the dark, to make mistakes 

 and entertain erroneous hypotheses which, with the increased know- 

 ledge of fact due to the work of a vastly increased body of 

 observers, give way to new conceptions in accordance with our 

 improved understanding of the phenomena before us. 



The Hertwigs failed to recognise the existence of the true 

 " ccelom " in Mollusca, viz. the pericardial, perigonadial, and renal 

 sacs. Further, they did not recognise that the cavitary system, which 

 they called " pseudocoel " in Mollusca (with, it is true, considerable 

 reservation as to its actual nature), is merely the blood-vascular 

 system in a swollen condition. They also associated under the 

 name " pseudocoel " various spaces in other animals which have 

 nothing in common with one another or with the haemoccel of 

 Mollusca. Lastly, they maintained (as it now appears erroneously) 

 the coelomic nature of the hsemocoel of Arthropoda as taught by 

 Haeckel and Gegenbaur, and as at that time accepted by me. 



The Hertwigs, in the historical retrospect at the close of their 

 volume Die Coelomtheorie, pay generous tribute to the work of Eng- 

 lish anatomists in establishing a true theory of the coelom. They 

 say : " Wahrend in England, wie uns der geschichtliche Ueber- 

 blick gezeigt hat, die Entdeckungen von Agassiz, Metschnikoff und 

 Kowalewsky auf einen fruchtbaren Boden gefallen waren und Mor- 

 phologen wie Huxley, Lankester und Balfour zu weittragenden 

 und zum Theil gliicklichen Speculationen veranlasst hatten, ist auf 

 diesem Gebiete in Deutschland keine Bewegung in das Leben 

 gerufen und eine Weiterbildung der besprochenen Theorieen nicht 

 versucht worden." 



(b) Progress in the Understanding of the Ccdomfrom 1881 to 1896. 



Whilst the conception of the coelom as essentially an entero- 

 coelous pouch, nipped off from the archenteron, is admitted to be 



