THE EGG-CELL OR OVUM. 



93 



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 verv certain, and that is that they are in no dis- 



FlG. 24. — Animal Cell, showing the chromatin elements of nucleus (a) in a 

 long coil, and the protoplasmic network (b) round about.— From Carnoy. 



order, but really preserve a very thorough defmiteness. Whether the 

 coil be continuous, as Van Beneden and others describe, or inter- 

 rupted, as Boveri and others maintain, is subsidiary to the more strik- 

 ing 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 behavior is so like that of minute independ- 

 ent 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 substance too that the germ-plasma, on which Weismann 

 and others have so much insisted, has its seat. 



II. Growth 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, 



