202 THE BEGINNINGS OF LIFE. 



suspended in fluid ; but a little later, as they come to 

 collect round the germinal vesicles, they are united 

 together in a mass by a firmer but clear basement sub- 

 stance, and when the minute ova have somewhat in- 

 creased in size, the outline of this clearer basement 

 substance of the yolk is distinguishable. There is not, 

 however, at first any external or vitelline membrane ; 

 of this Dr. Nelson and I have convinced ourselves by 

 repeated observations in Ascaris mystax. . . . The 

 ova, as they continue to descend in the vitelligenous 

 part of the tube in immense numbers closely pressed 

 together, assume the form of subtriangular flattened 

 bodies. ... A prodigious number of ova are thus 

 packed together in a very small space.' In many 

 instances it is only after fecundation has taken place 

 that the vitelline membrane seems to become de- 

 veloped. The production of this is usually spoken of 

 as the third stage in the formation of the ovum. In 

 all the simpler kinds of ova it is supposed to result 

 after the fashion of the cell-wall generally from c the 

 consolidation of the superficial part of the basement 

 substance ' of the yolk l . 



the yolk-substance, when it first begins to be formed, is scarcely granu- 

 lar, and in some instances quite clear, consisting of a viscous blastema. 

 . . . Very soon, however, and in many animals indeed from the 

 first, fine opaque granules make their appearance, as if by precipitation or 

 deposit in the clearer basement substance, and thus the primitive yolk-sub- 

 stance of the ovum in all animals is formed.' 



1 This mode of formation of the ovum in Ascaris corresponds 

 with the mode of origin of cells described by the upholders of the 

 'investment theory' (Umhullungs-theorie). 



