THE GASTRyEA THEORY AND ITS SUCCESSORS. IO5 



dseum, seem to indicate, however, that the Hydrozoa 

 and Ctenophores have Uttle in common and must be 

 regarded as two widely divergent stocks. If this be so, 

 the occurrence of an ectodermal origin for the reproduc- 

 tive cells in these two groups cannot be regarded as a 

 primary arrangement, but for some unexplained reason 

 has been secondarily acquired. We have seen that in 

 certain lower Crustaceans the " Keimplasma " may be- 

 come very early specialized, and immigrate quite inde- 

 pendently of the mesoderm and endoderm, the other 

 constituents corresponding to the central cells of the 

 Coelenterate planula. We may perhaps conceive of a 

 somewhat similar segregation of the germ-plasm occur- 

 ring in the Hydrozoa and the Ctenophores, the cells 

 containing it, however, remaining in the ectoderm, and 

 not migrating with the other cells, which are to form 

 the mesogloea cells and the endoderm. 



In conclusion I must express my admiration of the 

 masterly manner in which Metschnikoff has treated the 

 exposition of his theory. Probably no one has such an 

 acquaintance with the embryological phenomena of the 

 Coelenterates, or has contributed so extensively to our 

 knowledge of these phenomena, as Professor Metschni- 

 koff, and few possess that insight into the bearings of 

 facts which he has exhibited in all his studies. The 

 Parenchymella theory places our ideas of the relation- 

 ships of the Protozoa and Metazoa upon an entirely new 

 basis and enables us to overcome many great and per- 

 plexing obstacles. It is founded on facts and explains 

 them satisfactorily. 



The ideas advanced in this lecture which are not em- 

 bodied in Metschnikoff' s statement of his theory do not 



