84 MARINE BIOLOGICAL LABORATORY. 



note of which is the formation of the endoderm primarily 

 by delamination, invagination being thus a secondarily 

 acquired phenomenon. Thus Lankcster's views stand 

 in direct contrast to those of Haeckel. 



Like Haeckel, Lankester attempts^ to give the dif- 

 ferent historical phases recapitulated in ontogenetic 

 development. He starts with the ovum, the Monoplast, 

 corresponding to Haeckel's second stage, the Cytula. 

 This is succeeded by ih.Q Polyp last, equivalent to Haeck- 

 el's third and fourth stages, for Lankester recognizes two 

 different forms of Polyplast, — one in which it is solid, 

 the Morula, and another in which it is hollow, the blast- 

 ula, derived from the morula in the manner indicated by 

 Haeckel. Up to this point the only difference between 

 the two authors is the omission by Lankester of the 

 Monerula stage. To the Polyplast succeeds the diplo- 

 blastic Planula, for which Salensky's more convenient 

 term DiblasUda may be employed. This is a two-layered 

 vesicle without a mouth, the inner layer of cells (endo- 

 derm) having been formed by delamination. The cavity 

 of the blastula {blastoccel, Huxley) has now become the 

 digestive cavity, or archenteron, the cells lining it having 

 gradually acquired a digestive function while they still 

 formed the inner ends of the blastula cells, the acquisi- 

 tion of the function leading to their differentiation from 

 the non-digestive or ectodermic portion. 



Delamination is, however, a relatively unusual occur- 

 rence, invagination replacing it in a large number of cases. 

 How can its occurrence be explained on the basis of 

 the Planula theory 'i The changes which led to the 



^ E. Ray Lankester. Notes on Embryology and Classification of the 

 Animal Kingdom, etc. Quart. Joiirn. Micr. Sci. vol. xvii. 1877. 



