50 



THE CELL 



differ considerably in their structure from the nuclei met with in 

 ordinary tissues, as may be seen from Figs. 28, 29, 30. 



Fig. 28 represents the immature egg of a sea urchin ; if it is ex- 

 amined when alive, an exceedingly coarse network of rather 

 thick isolated threads can be distinguished. These, as is shown 

 by their micro- chemical properties, consist chiefly of linin. The 

 stained material is nearly all collected into a single large round 

 body, the " germinal spot" which lies in a net-knot of the frame- 

 work, where the greatest number of linin threads intersect. 



In the enormously large germinal vesicles, for which the large 

 eggs of Fishes, Amphibians, and Reptiles, which are so rich in yolk, 

 are remarkable, the number of germinal spots increases consider- 



kj 



-kn 



FIG. 28. FIG. 29. 



FIG. 28. Immature egg from the ovary of an Echinoderm. Ju the large germinal vesicle 

 there is a network of threads, the nuclear net, in which the germinal spot can be seen. 

 (O. Hertwig, Embryology, Fig. 1.) 



FIG. 29. Germinal vesicle of a small immature egg from the Frog. In a dense nuclear 

 net (fcn) a very large number of germinal spots, mostly peripheral (fc/), are to be seen. 

 (O. Hertwig, Embryology, Fig. 2.) 



ably during the growth of the cell, until finally they may number 

 some hundreds ; whether this multiplication takes place by division 

 or in some other fashion is not yet known. The position of the 

 germinal spots varies at different times ; generally, however, they 

 are situated on the surface of the vesicle, being distributed at even 

 distances over the membrane, as is shown in Fig. 29, where the 

 nucleus of a rather small immature egg of a frog is depicted. 



The shape of the germinal spots also varies ; they may be round 

 this is especially the case when they are isolated or oval ; some- 

 times they are somewhat extended, at others they are constricted 

 in the middle; occasionally they are irregular in outline, and when 



