200 THE BEGINNINGS OF LIFE. 



contrary, we are presented with typical instances of 

 their origination in the midst of the more or less solid 

 tissue of the ovary. 



The development of ova has been studied perhaps 

 with the greatest success amongst members .of the order 

 Nematoidea } for, on. account of the simplicity and trans- 

 parency of the ovarian tubes, the whole process of egg 

 formation can be watched in these animals more readily 

 than in many others. As pointed out by Dr. Nelson 1 

 and by Prof. Allen Thomson 2 , the process of egg de- 

 velopment commences in the Ascarides^ or Round 

 Worms, by the appearance c of minute cell-germs in 

 the upper part of the ovarian tube, immediately ad- 

 joining its caecal termination. 5 Leaving aside all 

 theories as to the precise mode of origin of these 

 ( cell-germs,' since this is a question on which we 

 possess no decisive evidence it is admitted by Dr. 

 Thomson himself that c some from the highest part are 

 mere molecules* although others a little further down 

 have already assumed the appearance of minute nucle- 

 ated cells. These nucleated cells constitute the so- 

 called c germinal vesicles.' Concerning the remain- 

 ing steps in the formation of the ovum in these animals 

 there is the greatest unanimity of opinion amongst 

 anatomists; so that, although we avail ourselves of 

 the description given by Dr. Allen Thomson, it must 



1 ' Reproduction of the Ascaris mystax,' in Philosophical Transactions, 

 1852. 



1 ' Cyclopaedia of Anatomy and Physiology,' vol. v. 1859, P- I2 - 



