Notes on some Hydromedusae from the Bay of Naples. 569 



Accordili^- to Metschxikoff ' a true morula is formed. 'The 

 cells of tlie morula appear to be identical in structure and cou- 

 sequently to show no dift'erence between ectoderm and entoderm. 

 Only after the number of embryonal cclls has bccome quite con- 

 siderable do the elemeuts lying at the periphery form a connected 

 layer, which is differentiated from the inner parenchyma mass by 

 a Sharp contour.' 



Brauer maintains that cleavage results in a coeloblastula, the 

 entoderm arising by multipolar delamination from the cells of the 

 blastula. While the embryo at this stage appears morula-like, he 

 maintains that it is really a bilamellar embryo, with both germ 

 hiyers established, though not yet sharply differentiated. 



So far as my own observations go they tend to confirm the 

 conclusions of Metschnikoff, though in some degree agreeing also 

 with phases of Brauer's conclusions. I bave found no evidence in 

 Support of the view that the germ layers are established during 

 early cleavage, as Brauer has claimed, nor that they result chiefly 

 from a delamination of any sort, whether primary or secondary, 

 unipolar or multipolar. That a morula-like condition often re- 

 sults from the more regulär type of cleavage there can hardly be 

 serious doubt. On the other band it is often just as evident that 

 a syncytium is formed, especially in cases of nuclear proliferation. 

 Here the differentiation of the germ layers is brought about by a 

 slow and graduated process of cell Organization and adjnstment. 

 The conditions found in this species resemble in many respects those 

 which I bave described in Eudendrium and Pennaria. For a con- 

 siderable time after the ectoderm is fairly established, the entoder- 

 mal mass continues as a more or less homogeneous aggregation of 

 cells in an undifferentiated condition. Finally the cells adjacent to 

 the ectodermal lamella assume a more or less columnar shape, form 

 a continuous series, and thus gì ve origin to a definitive entoderm. 

 Coincident with these changes, or shortly following, the inner mass 

 of cells show signs of disintegration, become vacuolated and tend 

 to liquefy, thus giving rise to the gastric cavity of the embryo. 



Later History of the Embryo. 



Concerning the formation of the actinula there is little to be 

 added to the account given bv Ciamician. The embrvo becomes 



Embryolog. Studien an Medusen 1886 pag. 69. 



