140 Dr. O. Zacliarias on the Reproduction and 



(including the bud) in this stage acquired the form of a 

 rosette (fig. 4, 4), and gave origin simultaneously to five 

 daughter-nuclei. In the formation of the other blastomeres 

 the origin of the daughter-nuclei belonging to them from the 

 original germinal vesicle may almost regularly be observed. 



It is worthy of mention that in the egg of Philodina roseola 

 the occurrence of a small segmentation-cavity may be defi- 

 nitely recognized. J have been able to recognize this structure, 

 not only in isolated cases , but in all the eggs of which I wit- 

 nessed the first stages of development. 



After the formation of the first smaller segment, the seg- 

 mentation advances as follows : — Two other blastomeres are 

 produced from the larger half of the egg, take the original 

 first divisional piece between them, and become amalgamated 

 with this superficially. 



As in Rotifer vulgaris, so also here, we observe that the 

 first three blastomeres now also divide and begin to grow 

 round the hypoblastic half of the egg. In the latter, in the 

 meanwhile, more vitelline granules have accumulated than in 

 the foundation of the epiblast, by which means, as we shall 

 see, it becomes possible to keep the hypoblastic cell in view 

 during the subsequent development. By degrees the still 

 unsegmcnted very granular portion of yelk is regularly flowed 

 round by the dividing blastomeres, and surrounded as with a 

 hood. But before the blastopore of the latter is completely 

 closed two smaller portions separate off from the hypoblast, 

 and these are distinctly marked by the great abundance of 

 granules they contain. They are of a deep red colour, and lie 

 just within the blastopore. I do not hesitate to interpret 

 these red cells as the rudiment of a mesoblast, from which the 

 ovaries, the muscles, the excretory vessels, and the clavate 

 organ (in the foot) originate. 



The developing mesoblastic rudiment does not form a layer, 

 but only a cell-cord, which grows from each side towards the 

 anterior end of the body (between the epiblast and hypo- 

 blast). It is impossible to trace this growth in detail, because 

 finally the larger hypoblastic cell begins to divide, and then 

 the differentiation of the middle from the inner lamella 

 becomes illusory. But by turning the egg upon its long axis 

 we see that the mesoblastic cell-cord does not take a central 

 coarse, but is more approximated to one half of the egg than 

 to the other. 



I do not think I am mistaken in regarding the first-men- 

 tioned red cells as those u primordial cells of the mesoderm," 

 the occurrence of which in the development of the Annelida 



