DEVELOPMENT OF FLUSTRELLA HISPIDA. 



435 



On the Early Stages in the Development of 

 Plustrella hispida (Pabriciiis), and on the 

 Existence of a " Yolk Nucleus " in the Egg 

 of this Form. 



By 



R. M. Pace (nee Clark), 



Late Scholar of Girton College, Cambridge. 



With Plates 22—25. 



Contents. 



Introduction ...... 



Material and Methods 



Spermatogenesis and Oogenesis 



The Maturation of the Egg : the Yolk Nucleus 



The Nature and Function of the Yolk Nucleus . 



The Yolk Nucleus and Yolk Formation in Bryozoan 



Eggs ..... 



Fertilisation, and the Passage of the Egg into the Tentacle 



Sheath ...... 



Segmentation, and the Formation of the Germinal Layers 



Summary ..... 



Comparison with other Bryozoa . 

 The Development of the Larval Organs 

 The Mature Larva ..... 



The Degeneratnig Larva .... 



Tiie Alimentary Canal in Larval Ectoprocta . 

 General Summary ..... 



References ...... 



Explanation of Plates .... 



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Introduction. 



In the following paper an attempt has been made to trace 

 out the earlier stages in the development of Flustrella 

 hispida. The larva of this familiar Bryozoan has previously 



