29G THE GERM-LAYEKS AND EVOLUTION 



But it was not Haeckel himself who enunciated the ccelom 

 theory. 



It will be remembered that Remak introduced in 1855 

 the conception of the mesoderm as an independent layer 

 derived from the endoderm. The pleuro-peritoneal or 

 body-cavity was formed as a split in the " ventral plates " of 

 the mesoderm. Haeckel's " coelom " corresponded to the 

 " pleuro-peritoneal cavity " of Remak, but his view of the 

 origin of the mesoderm brought him much closer to von 

 Baer's conception of the origin of two secondary layers from 

 ectoderm and endoderm respectively than to Remak's 

 conception of the mesoderm as a single independent layer. 



Much uncertainty reigned at the time as to the exact 

 manner of origin of the mesoderm; 1 some held that it 

 developed from the ectoderm, others that it originated in the 

 endoderm, while still others, and among them Haeckel, 

 considered that part of it came from the ectoderm and part 

 from the endoderm (pp. 23-4, 1874). 



The solution of the problem came from those observa- 

 tions on the development of the lower forms to which we 

 have just alluded. 



The early history of these discoveries and of the theory 

 which grew out of them has been well summarised by 

 Lankester, 2 and may conveniently be given in his own 

 words : 



"As far back as 1864 Alexander Agassiz (" Embryology 

 of the Star-fish," in Contributions to the Natural History of 

 the United States, vol. v., 1864) showed in his account of the 

 development of Echinoderma that the great body-cavity of 

 those animals developed as a pouch-like outgrowth of the 

 archentcron of the embryo, whilst a second outgrowth gave 

 rise to their ambulacral system; and in 1869 Metschnikoff 

 (A ft in. dc l } Acad. i 'nipt 'n ; alc dcs Sciences dc St J\ : tcrsl>onrg, 

 series vii., vol. xiv., 1869), confirmed the observations of 

 Agassiz, and showed that in Tornaria (the larva of 

 Balanoglossus) a similar formation of body-cavities by 

 pouch-like outgrowths of the archenteron took place. 



1 See Balfour, he. cit., Chapter xiii. 



- A Treatise on Zoology, Pt. ii., 1900. Introduction by Sir E. Ray 

 Lankester. 



