BROOKS AND RITTENHOUSE: ON TURRITOPSIS. ' 453 



The formation of the germinal layers in Turritopsis is different 

 from that which has generally been described for the development of 

 hydromedusae. In the majority of forms previously studied the 

 differentiation took place either by delamination or by cellular ingres- 

 sion, unipolar or multipolar. These methods have been well de- 

 scribed and figured by Metschnikoff. 



In Aglaura and Rhopalonema there is found according to Metsch- 

 nikoff a solid so called morula stage destitute of cleavage cavity, the 

 superficial cells of which are converted into the ectodermal layer, 

 while those within represent the endoderm. Here the two layers are 

 formed directly without the formation of a syncytial structure. 



In Eudendrium and Pennaria according to Hargitt's description, 

 a condition somewhat similar to that of Turritopsis is found. He 

 says: "Indeed in both Eudendrium and Pennaria, not to mention 

 other cases, cleavage would seem to result primarily in the formation 

 of a more or less characteristic syncytium, the subsequent development 

 of the germ layers taking place by a gradual differentiation of the 

 syncytial elements, first and naturally the ectoderm, and later, often 

 very much later, the endoderm." 



The syncytial character in Turritopsis is acquired under favorable 

 conditions, when the embryo is about six hours old, at the time that the 

 irregular mass of segmenting cells is metamorphosed into the oval 

 embryo. I am inclined to think that the formation of the syncytium 

 and the change of shape of the developing embryo are connected 

 phenomena. The length of time during which this condition lasts 

 is evidently comparatively short, for soon cilia develop and the larva 

 begins to swim ; but meanwhile the peripheral region of the syncytium 

 has been transformed into a distinct layer of ectoderm cells, separated 

 from the inner mass of tissue, still structureless in character, by the 

 development of the mesogloea. 



From the fact that a syncytium or plasmodium-like structure is 

 formed, it is impossible to localize any of the blastomeres of the seg- 

 menting egg which will form special parts of the future embryo. Even 

 those cells which are at the surface at the completion of segmentation 

 cannot be regarded as primitive ectoderm, for in the breaking down 

 of the cell boundaries, the formation of the syncytium, and the recasting 

 of the cells, it is quite impossible to say what changes of the proto- 

 plasm may take place. 



