12 



DEVELOPMENT OF THE FROG'S EGG 



[Cn. 



Centrosomes can generally be demonstrated in these resting-cells. 

 Other cells have the nuclei broken up into a number of smaller 

 spheres. Still other nuclei may have a deep depression on one 

 side, as though the nucleus were dividing into two by constric- 

 tion. These nuclei have often been described as dividing by 

 amitotic or direct division, i.e. without the characteristic mitotic 

 division. Meves ('91) has stated that in the testes of the sala- 

 mander amitotic division occurs regularly, and he believes that 

 such cells will later form spermatozoa. Other authors (Bellonci 

 and vom Rath), admitting that such a division may take place, 

 affirm that such cells are in process of degeneration, and never 

 subsequently form spermatozoa. Vom Rath declares that cells 

 that have once divided by amitosis can never again divide by 

 karyokinesis (mitosis), and that such cells degenerate later, and 

 do not ever develop into sex-cells. 



Oogenesis 



The origin of the egg in the ovary of the frog has been 

 studied by Schultze ('87, e), but many important details are 



still unknown. The egg 

 derived from one of the 

 cells of the outer layer of 

 the ovary is surrounded 

 by a large number of fol- 

 licle-cells. The nucleus 

 of the egg consists of fine 

 chromatic threads, of a 

 nuclear sap, and of scat- 

 tered nucleoli. As the 

 egg enlarges the nucleus 

 also enlarges, and the 

 chromatin stains more 

 faintly and appears in the 

 form of scattered threads. 

 The nucleoli stain well 

 and become larger and more numerous as the egg enlarges, 

 and are found generally around the periphery of the nucleus. 

 Certain portions of the protoplasm now begin to stain dif- 



Fig. 8. — Ovarian egg of Rana. 



