THE FUNCTION OF THE CENTROSOME. 



207 



life-cycle. The cells constituting the Malpighian layer of the 

 epidermis in vertebrates and the apical "budding zone" in 

 certain annelids continue to divide throughout life. Other 

 cells, in which for a long time karyokinetic activity is sus- 

 pended, may upon occasion undergo rapid division; e.g., the 

 cells which give rise to the temporary ovaries in some of the 



Primordial 



Germ-cell 



Oogonia - • 

 O'dgoitia . . 

 O'dgonia - ■ 



A W 



---• • • • 



A /I /\ l\ 



Division-Period 



Primary Oocyte 



Secondary Oocyte 

 Egg 



Gro^vth -Period 



/ \ 



\ 



• • 



/\ 



'Pg' 



-Pg- 



Maturation- Period 



Fig. 3. — Diagram showing the development of the mature egg from the primordial 



germ-cell (after Boveri). 



lower invertebrates and the cells which commence the resrener- 

 ation of lost tissues. Many cells also, e.g., the neuroblasts, 

 cease dividing permanently. 



I wish, however, particularly to call attention to the pecul- 

 iarities in the rate of cell-division exhibited towards the close 

 of the life-cycle by the cells which give rise to the mature 

 ovum. After a long series of consecutive divisions of the 

 oogonia (Fig. 3), a generation of cells arises in which 

 karyokinesis is, for a time, suspended. These cells are the 

 ?^«maturated eggs — the ^^ primary oocytes!' The oocyte, after 

 enjoying a period of rest and growth {which may sometimes be 

 measured in months or even in years), sooner or later divides 

 into a small cell — the first polar globule, and a large cell — the 

 secondary oocyte. The latter also divides to form a small cell — 



