DEVELOPMENT OF PARA VORTEX GEMELLIPARA 503 



on to each of the daughter cells. It is possible, on the other 

 hand, that the division is effected directly by the traction of 

 the spindle fibers upon the mass. Very likely both factors 

 act simultaneously. 



Certain it is that beyond the stage where the mitochondrial- 

 mass appears in two or more blastomeres the centrosome lies 

 in its center. Previously the centrosome was difficult to detect 

 among the numerous granules in the cytoplasm, but now that 

 it has come to lie in a more finely homogeneous substance, it 

 is conspicuous. Figures 14 and 16 plainly show it in a clear 

 region at the center of the mitochondrial mass. Had its his- 

 tory not been carefully traced the picture here presented would 

 suggest that the whole structure answered to the centrosphere 

 containing a centriole. That the black dot represents more 

 than a mere centriole is indicated by its further behavior. Figure 

 19 shows that the solid dot becomes enlarged as the new spindle 

 forms, and contains a light center. Ultimately the body assumes 

 the form of a sphere whose shell consists of a multitude of tiny 

 granules, and at whose center lies the true centriole. Further- 

 more, it lies outside the mitochondrial mass in one cell of figure 

 20. 



c. Blastula. It has been noted that no distinct segmentation 

 cavity is formed in the blastula. The conditions at this stage 

 are well shown in figures 14 and 16 which represent sections of 

 different embryos, and in figure 15, a drawing of a living embryo 

 which, with a small mass of yolk granules, was freed from its 

 capsule. The blastomeres lie in rather close contact with each 

 other, with the exception that a cap of cells which have received 

 the portions of the original yolk-nucleus tends to become sepa- 

 rated from the mass of embryonic cells which is to give rise to the 

 living tissues of the embryo. A solid morula is, then, typical of 

 P. gemellipara as of Turbellaria in general and many other lower 

 invertebrates. But for the cells containing the mitochondrial 

 mass this morula closely resembles that of Hydra and other 

 Coelenterates. Bresslau figures for Mesosotomum ehrenbergi a 

 morula somewhat less compact and more irregular in outline. 



