THE EGG-CELL OR OVUM. 99 



When Purkinje, in 1825, discovered the nucleus of the 

 fowl's egg, he could have little idea that the little "vesicle" to 

 which he directed the attention of investigators was in reality an 

 intricate microcosm. Little more than ten years elapsed, before 

 R. Wagner began to complicate matters by the discovery of the 

 nucleolus or germinal "spot" within the "vesicle." We now 

 know that the nucleus has not only a very complex structure, 

 but in a sense a curious internal life all its own. 



The nucleus, when quiescent, often lies in a little nest or 

 chamber within the cell-substance, and is limited from the latter 

 by a more or less distinct nuclear membrane, which disappears 

 as the period of activity begins. Inside this membrane, it is 

 often possible to distinguish one or more of the aforesaid 

 nucleoli, lying in a more fluid material often called the " nuclear 

 sap." About these nucleoli and bodies more or less like them, 

 about the reasons for their variable number and form, very little 

 that is certain can be said. Much more important is the 

 essential constituent of the nucleus, a system of strands, coils, 

 or loops, which stain deeply with various dyes, and are there- 

 fore known as the chromatin elements. In contrast thereto, the 

 less stainable and less essential constituents of the nucleus are 

 distinguished as achromatin. 



The chromatin elements in the resting nucleus are oftenest 

 arranged in a manifold coil, like a disordered ball of twine, 

 while in other cases they appear rather as a living network. 

 One thing about them seems very certain, and that is that they 

 are_in no disorder, but really preserve a very thorough definite- 

 ness. Whether the coil be continuous, as Van Beneden and 

 others describe, or interrupted, as Boveri and others maintain, 

 is subsidiary to the more striking fact, that in the state of activity 

 the number and disposition of the dislocated or loosened parts 

 of the coil remain definite and orderly, and that their behaviour 

 is so like that of minute independent individualities that any 

 rough-and-ready account of the mechanics of cell division must 

 at once be ruled out of court. It is within the chromatin sub- 

 stance too that the germ-plasma, on which Weismann and others 

 have so much insisted, has its seat. 



§ 2. Groivth of the Ovum. — When the ovum is very young, 

 it very generally presents the features of an amoeboid cell. In 

 some cases this phase persists for a longer time, as in the ovum 

 of hydra, which in all essentials is comparable to an amc^ba. 

 Even in the simplest animals, however, the amoeboid phase 



