46 THE CELL 



retained, the nucleus appears as a compact globular mass of 

 nuclein ; this is the case in the sperm elements of Ascaris 

 megalocephala (Fig. 22), which, when immature, are shaped like 

 fairly large, round cells, and when mature 

 assume the form of a thimble. 



Having examined this simple condition 

 of the nucleus, as it occurs in sperm-cells, 

 b and where it is composed almost entirely 



FIG. 22. Sperm-ceil of o f active nuclear substances, being nearly 

 yrr B ~T,ra * from the admixture of other sub- 

 Hertwig's Embryology, Fig. stances, we may now proceed to examine 

 c'^ * w!S ! H S <**"* nuclear forms. In these TO see that 

 itself to the egg; /shining the chief cause for the variety in form, which 



substance resembling fat. ^ been observed in ^^ and an i ma l ce U S , is 



the fact, that the active nuclear substances evince a great inclination 

 to take up liquid, with the substances dissolved in it, and to store it up, 

 generally to such an extent, that the whole nucleus acquires the ap- 

 pearance of a bladder enclosed in protoplasm. 



Thus in the nucleus, a process takes place similar to that which 

 occurs in protoplasm, where the cell-sap becomes collected in 

 vacuoles or large sap-cavities. This circumstance bears the same 

 significance in either case. These vacuoles are concerned in the 

 metabolism both of the cell and of the nucleus, for they contain 

 in solution nutrient materials, which can be easily taken up by 

 the active substances, in consequence of the great superficial de- 

 velopment of the vacuoles. 



This process of sap absorption may be directly observed when, 

 after fertilisation has taken place, the nucleus of the sperma- 

 tozoon, in performance of its function, enters the egg-cell. In 

 many cases it begins to swell up gradually, until it becomes ten 

 to twenty times as large as it was originally ; this is not due to 

 any increase of its active substances, which remain absolutely 

 unaltered in quantity, but entirely to the absorption of fluid 

 substances which were held in solution in the yolk. In such a 

 nucleus, which has become transformed into a vesicular body, the 

 nuclein is spread out in fine threads to form a net ; in addition, 

 one or two globules of paranuclein (nacleoli) are now to be seen. 

 A similar process occurs each time a nucleus divides, when the 

 daughter nuclei are being reconstructed. 



According as to whether the nucleus has absorbed a greater or 

 less quantity of nuclear sap, its solid constituents, which on account 



