THE CELL 



continue to live for a time, but it can neither grow nor undergo progres- 

 sive differentiation. All changes in enucleated protoplasm are regressive, 

 leading to death. The nucleus is also largely the reproductive center, as 

 will be described below. The nucleolus plays the role, among other pos- 

 sible functions, of a center of storage, perhaps also elaboration, of chro- 

 matin. Xuclear protoplasm, more especially the chromatin, is relatively 

 rich in phosphorus. 



Astral System. Another organ of a typical cell is the aster, astral 

 system or attraction sphere. Its substance is collectively known as archo- 

 plasm. It usually lies outside of, 

 but close to, the nucleus; in cer- 

 tain cells it is intranuclear, e.g., 

 spermatocytes of Ascaris. It con- 

 sists centrally of a granule, the 

 centrosome; in this, in certain in- 

 stances, may be differentiated cen- 

 trally a smaller granule, the cen- 

 triole; when the latter appears, 

 the more outlying portion of the 

 centrosome is designated the cen- 

 troplasm. The centrosome may 

 divide into two, becoming a diplo- 

 some, or in some instances it 

 may become multiple, when it is 



FIG. 6. EGG OF A CLAM (CUMINGIA 



TELLINOIDES). 



Showing the first maturation spindle 

 with centrosomes and chromosomes at 

 metaphase, and the disappearing nucleojus 

 (ri) at the right. X 1000. 



known as a pluricorpuscular cen- 

 trosome. Surrounding the cen- 

 trosome is a clearer, minutely 

 granular sphere, the centrosphere ; 

 radiating from this peripherally are delicate astral rays, collectively 

 known as the astrosphere (Fig. 6). Structurally the aster is subject to 

 considerable variations in different cells. On account of its relation to 

 cell division, it is regarded as the dynamic center of the cell ; viewed thus 

 its substance is known as Jcinoplasm. The attraction sphere may or may 

 not be visibly present ; in all living cells it, or its analogue, is generally 

 believed to be potentially present. 



Cytoplasm. The cytoplasm may be divided into a thin peripheral or 

 cortical layer of less granular protoplasm, the exoplasm (ectoplasm), 

 and the main central mass, the endoplasm. In certain highly differen- 

 tiated cells the exoplasm is not discernible. In others, at certain stages 

 m the development it contains the products of differentiations, when it is 



