138 THE EVOLUTION OF MAN. 



afterwards grows so considerably that it expands to the 

 well-known yellow ball of yelk. The nucleus, or the germi- 

 nal vesicle, of the egg-cell, is thus pressed on to the surface 

 of the spherical cell, and is there embedded in a small mass 

 of clear, so-called white yelk. This then forms a circular 

 white spot, which is called the tread, or cicatricle (cica- 

 tricula, Fig. 12, 6). From the tread a thin cord of white 

 yelk passes through the yellow to the middle of the round 

 cell, where it swells to a little, central ball, the falsely-called 

 yelk-cavity (latebra, Fig. 12, d}. The yellow yelk, which 

 surrounds this white yelk, appears in the hardened egg 

 in concentric layers (c). The yellow yelk is encircled by 

 a delicate structureless yelk-skin (membrana vitellina, a). 



Of late it has been widely believed that the large yellow 

 egg-cell of the Bird, which in the case of the largest birds 

 reaches a diameter of several inches, cannot be regarded as 

 a simple celL But, with Gegenbaur, we believe this view 

 to be erroneous. The unimpregnated and unsegmented egg- 

 cell of the Bird, with its simple nucleus, remains a simple 

 cell, even though its yellow yelk-substance increases very 

 greatly. Every animal which consists of a single cell, every 

 Amosba, every Gregarina, every Infusorial animal, is one- 

 celled, and remains so, however much food of various kinds 

 it absorbs. In the same way the egg-cell remains a simple. 

 cell, however much food-yelk it may afterwards collect 

 within its protoplasm. Gegenbaur has proved this clearly 

 in his excellent work on the embryos of Vertebrates. 43 



The Bird's egg, of course, assumes a different form as 

 soon as it is fertilized. Its germinal vesicle, or nucleus, then 

 separates by repeated division into many parts, and the 

 protoplasm of the tread, which surrounds it, is corre- 



