fli 



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 Ascarls 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 



... A prodigious number of ova are thus 



bodies. 



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 Hhe 

 consolidation of the superficial part of the basement 

 substance ^ of the yolk i. 



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

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

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

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



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



F 



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 1^^ 

 ' investment theory ' (Umhiillungs-theorie). 



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Kajrammatic repres 

 Orisacs in differe 



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 (■ f eritoneal 



■^°^^riaa stroma, 

 ^^^t^o layers of 



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