264 T. IWAKAWA. 



wliicli, as he acknowledges, represents a rare instance, 

 ■svould seem to favour his theory of the origin of the ova, 

 according to which entire epithelial cells, in larger or smaller 

 numbers, sink into the stroma, and are subsequently over- 

 grown by the closing up of the epithelium. I have done my 

 best to find evidence to support this view, but my observa- 

 tions have led me to a somewhat different conclusion, as will 

 be seen from the following pages. 



Origiji of the Otum. 



Before stating my own conclusions, I will give briefly 

 those which have been put forward by other authors. 



Van Beneden (5), from an extended and comprehensive 

 study of oogenesis, chiefly in invertebrate animals, came to 

 the conclusion that the ovum invariably arises from a 

 nucleated mass of protoplasin. 



"VValdeyer (4), starting from the other end of the animal 

 kingdom — the vertebrates, arrived at the conclusion that 

 the ovum of every egg-producing animal is at first an epithe- 

 lial cell. So far as the vertebrate animals are concerned, 

 this opinion has met with general acceptance. 



For Ludwig (6) the chief question is. Does the ovum re- 

 present a single cell ? His view may be stated in his own 

 words (6, p. 194): 



*^Our investigations have taught us that in all animals, 

 without a single well-established exception, the egg is a 

 single simple cell from the outset, and that it does not lose 

 this character before maturity." 



Gotte (7), on the other hand, has maintained that the 

 primordial ovum of Bombinator igneus does not represent a 

 single epithelial cell, but that several such cells coalesce, 

 and thus give rise to a mass of protoplasm, in which are 

 suspended the nuclei of the original cells. Later these 

 nuclei also coalesce, forming a single body. This uni- 

 nuclear mass does not, according to Gotte's interpretation, 

 represent a cell, but an unorganised body, which at this 

 stage contains no ^' proper egg material,^' i. e. contains no 

 yolk-substance, which alone is the foundation [" Erzeuger 

 der spateren Entwickelung zum selbststandigen Leben!") 

 of all vital development. 



The recent investigations of Dr. Nussbaum (8) have 

 placed this matter in a new light, and have opened up ques- 

 tions of great interest, which can only be decided by embryo- 

 logical studies. 



Dr. Nussbaum finds the functional part of the genital 

 organs of Ra7ia fusca, in a larva which has attained the 



