64 CYTOLOGY CHAP. 



the chromosomes in the germinal vesicles of many animals has similarly 

 been held to be an expression of the intense activity required of them in 

 connection with the elaboration of the yolk. It has also been considered 

 that the enormous mass of nucleolar substance present at this stage 

 represents merely the accumulation of waste products of great metabolic 

 activity. Finally, many cytologists have described the actual extrusion 

 of chromatin from the germinal vesicle into the cytoplasm, either directly 

 from the chromosomes or indirectly by way of the nucleolus. The 

 extruded chromatin (chromidia) is supposed to take part in yolk forma- 

 tion, either by direct transformation into this substance, or by exerting 

 a formative influence on the cytoplasm. This matter of the extrusion 

 of chromatin is dealt with more fully in Chapter VI. 



We are thus introduced to a body known as the yolk nucleus (not to 



be confuse^ with the em- 

 bryologist's yolk nuclei of 

 Selachian, etc., embryos, 

 which are derived from 

 supernumerary sperma- 

 tozoa ; see p. 77). Dur- 

 ing the early growth 

 period, intensely staining 

 granules .appear in the 

 cytoplasm of the oocyte. 

 These are variously inter- 

 preted as extruded chro- 

 matin, or as chondrio- 

 FIG. 27. 



Yolk nucleus (y.n.) in the oocytes of (A) Aniedon biflda (Chubb, Phil. SOniCS (S66 Chapter V 1. J . 

 Trans., 1906), and (B) Paracalanus parvus (Morofif, A.Z., 1909). Sometimes, as in EchinUS 



(Schaxel, 1911 a) and Hydr actinia (Beckwith, 1914), they are 

 scattered uniformly through the cytoplasm. In other cases they are 

 concentrated into a more or less compact mass, often round the 

 centrosome as a centre, forming a conspicuous body in the cytoplasm. 

 Examples of such cases are found in Antedon (Chubb, 1906 ; Fig. 27), 

 certain Copepoda (Fig. 27), Amphibia, etc. The supposed connection 

 of these cytoplasmic bodies with yolk formation (whence their name of 

 " yolk nuclei ") rests chiefly upon the facts that their appearance 

 precedes yolk formation, and that as this proceeds they disintegrate 

 and finally disappear in the ripe oocyte, where no more yolk is being 

 deposited. 



The term yolk nucleus has also been applied to what is probably an 

 entirely different structure, namely, the centrosome and surrounding 

 substance of the centrosphere, which sometimes forms a large conspicuous 

 body (e.g., Enchytmeus, Vejdovsky, 1907). 



