THE BEGINNINGS OF LIFE. 



205 



Whether or not the first rudiments of the ovum, the 

 germinal vesicle, is formed first or formed within the 

 ovisac, must therefore still be considered an open ques- 

 tion, although the balance of evidence seems, perhaps, 

 rather more favourable than adverse to its secondary 

 formation ; and if this were the case, the process would 

 strongly resemble that by which the vegetable ovule 

 arises in all flowering plants. Turning, however, to 

 the question of the mode of formation of the ovum 

 itself, Dr. Allen Thomson tells us that the earliest 

 stages in its development are best traced in such 



FIG. 14. 



Portions of the Ovarian Stroma and Ovisacs of the 

 Thrush. (Thomson.) 



a. Earliest state of ova to be perceived in ovarian stroma, consisting, 



first of minute granular spots ; next of clear points within a granular 

 mass; and thirdly, of small germinal vesicles surrounded by the 

 minutely granular dark yolk-substance. 



b, c. Different stages of formation of the ovisac round the small ova : 



epithelium is seen to line the sac, and the germinal vesicle, with 



occasionally a single macula, is now apparent. 

 d. The ovisac and ovum in a more advanced stage, 

 of. Ovisac with epithelial lining. 

 v. Minutely granular yolk. 



