. Cytology, with special reference to the metazoan nucleus. Cells. 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 c
. Cytology, with special reference to the metazoan nucleus. Cells. 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 confused with the em- ⢠brvologist'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- ⢠. â preted as extruded chro- ⢠⢠matin, or as chondrio- , ^- u !^" ^lls A ,j Av-^./rhâhh Phil somes (see Chapter VI.). Yolk nucleus {) in the oocytes of (A) Anledon bifida (Chubb, FliU. \ r / Trans., 1906), and (B) Paracfl/anMS ^amis (Moroff.^.Z., 1909). SomctimCS, aS iu EclliflUS (Schaxel, 1911 a) and Hydractinia (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 yo
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