454 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 



FORMATION OF THE EXDODERM. 



The formation of the endoderm in Turritopsis cannot be adapted 

 to any of the schemes of the development of the hydromedusae that 

 have been sketched by Metschnikoff. He distinguishes three principal 

 methods for the development of the inner germ layer: first, delamina- 

 tion, a process in which the segmenting blastomeres divide in a plane 

 nearly parallel to the surface, and the inner parts or cells become 

 primitive endoderm, while the outer parts remain as primitive ecto- 

 derm. Second, multipolar ingression, in which cells migrate into the 

 blastocoele from different regions of the peripheral cell layer, and are 

 transformed into endodermal tissue directly. Of this mode he de- 

 scribes several subordinate types. Third, unipolar migration, similar 

 to the preceding except that the primitive endoderm cells are given 

 off at one pole only, at the posterior end of the larva. 



In Turritopsis the endoderm is derived from the syncytial mass of 

 tissue left in the center of the embryo after the ectoderm has been formed 

 and separated off by the development of the mesogloea. The inner 

 germ layer as a rule is formed much later than the ectoderm. Soon 

 after the supporting membrane is developed, cell boundaries begin to 

 appear in the syncytium in the interior of the larva. The cells thus 

 formed are primitive endodermal cells, and are crowded together 

 without any definite arrangement for a number of hours. Stages in 

 which the cell walls are reappearing are shown in figures 48, 49 (pi. 34) 

 and 50 (pi. 35). When the embryo is about forty-eight to sixty hours 

 old, the time at which attachment takes place, a fissure appears in the 

 middle of the mass of endodermal tissue. This is the beginning of 

 the coelenteric cavity. This separation begins near the anterior part 

 and grows toward the posterior end. The coelenteron gradually 

 increases in size, and at the same time the endodermal cells begin to be 

 rearranged, and finally become situated parallel to each other with 

 their bases against the mesogloea and form a definite inner germ layer. 



Gerd has observed in Bougainvillia that during the course of cell 

 multiplication the boundaries of the cells become indistinct and that 

 the peripheral and central nuclei are altogether identical. But this 

 species differs from Turritopsis, according to his description, in the 

 formation of the compact morula stage, in that it is brought about by a 

 multipolar migration of cells into the interior of the coeloblastula; 



