THE GASTR^A THEORY AND ITS SUCCESSORS. 93 



tion be explained ? Metschnikoff imagines that both 

 delamination and immigration have arisen simulta- 

 neously as a natural sequence of the modes of non- 

 sexual multiplication found in the Flagellata, where 

 transverse division and longitudinal division both con- 

 tribute, sometimes one sometimes the other occurring, 

 or, as in certain Chlamydo-monadinae, both processes 

 occurring together, colonies being thus built up. If in 

 the lower Metazoa longitudinal division predominates, 

 we get immigration, but if transverse division also oc- 

 curs delamination results. This seems, however, hardly 

 to explain what happens ; it does not explain why longi- 

 tudinal division, i.e. the division in a plane perpendicular 

 to the outer surface of the blastula, prevails exclusively 

 during the conversion of a large-celled blastula into one 

 with columnar narrow cells, the transverse division then 

 suddenly appearing. It seems quite possible to explain 

 the origin of delamination in another way. We know 

 that in many ova food material may be aggregated at 

 one pole, and if such a polar storing-up of food should 

 have occurred in the various cells of the blastula, or of 

 an ancestral form corresponding to it, it is easy to under- 

 stand how it would be to the advantage of the organism 

 for the inner portion of its cells to divide off, to delami- 

 nate, instead of migrating in toto. A certain amount of 

 migration might accompany this process, as in Coty- 

 lorJiiza and Aurelia, or the new delamination might 

 entirely replace the earlier process as in Renilla and 

 Metriditini, and if this specialization were carried a little 

 further, typical delamination such as occurs in Geryojiia 

 and Lii'iope would result, the embryo in these cases 

 being not a solid planula but a hollow diblastula. 



