9 2 



THE EVOLUTION OE SEX. 



CHAPTER VIII. 



THE EGG-CELL OR OVUM. 



TN the preceding chapter we sketched the history of the "ovum- 

 theory," which expresses the now familiar fact that every organism, 

 reproduced in the ordinary way, develops from a fertilized egg-cell. 

 It is now necessary to attend more carefully to the essential characters 

 and history of this " primordium commune," — this common starting- 

 point of life, — leaving the details, along with the other problems of 

 development, to a special volume devoted to Embryology. 



I. Structure of the Ovum. — The ovum presents all the essen- 

 tial features of any other animal cell. There is the cell-substance, con- 

 sisting in part of genuine living matter or protoplasm; and there is 

 the nucleus, or "germinal vesicle," which plays such an important 

 part in the ripening, fertilizing, and subsequent division of the cell. 



The cell-substance exhibits, when highly magnified, a homogene- 

 ous matrix, traversed by a delicate network, with minute yelk-balls, 

 pigment, and other granules strewn about the meshes. So much of 

 it is genuine protoplasm, of course, but then there are also substances 

 in process of ascent and even descent from the climax of living matter, 

 and there is in more or less abundance a reserve capital of yelk- 

 nutriment for the future embryo. Delicate observations, by the 

 modern masters of microscopic technique, have detected many mar- 

 vels in the egg-cell, into which we can not at present enter. Thus, 

 within the last year, Boveri has drawn attention to a special element 

 in the protoplasm, which he calls archoplasm, a substance which, as its 

 name suggests, seems to have an altogether marvelous architectural 

 function in relation to the changes of the nucleus in segmentation. 



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 investigaters was in reality an intricate microcosm. 

 Little more than ten years elapsed before R. Wagner began to com- 

 plicate 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 ol 

 activity begins. Inside this membrane, it is often possible to dis- 

 tinguish one or more of the aforesaid nucleoli, lying in a more fluid 



