The determination of sex in animal development. 721 



stated, that they were abundant ("recht häufig") in Rana esculenta. 

 Such large spermatozoa have, therefore, been described in the testis 

 of the frog and toad, whereas ripe eggs, or eggs anywhere near ripen- 

 ing, have never been seen there. From a partial search of the liter- 

 ature of spermatogenesis it is curious and very significant to note 

 how relatively often a second form of spermatozoon has been men- 

 tioned: the text-books naturally hardly ever refer to it, or, if they 

 do so, their notice is of the briefest description. Probably it has 

 hitherto appeared to be too mysterious an object for general mention ! 

 Without citing more recent cases, for to do so would serve no 

 useful object, one further example of an hermaphrodite male frog may 

 be noticed. It is the paper, rather than the frog, which is of interest. 

 The author, F. Friedmann ^), comments upon the "höchst seltsame Be- 

 hauptung" of Knappe*), that the so-called eggs of the testis oi Bufo, 

 if they develop further, yield spermatocytes and spermatozoa. Accord- 

 ing to the former, in face of our present knowledge this is "unhalt- 

 bar" ; for eggs, or oocytes, cannot yield spermatozoa, (notwithstanding 

 the current opinion, that eggs and sperms are completely homologous). 

 A theory may be made untenable by a fact, or by facts: a fact is 

 either true or false, and it cannot be influenced in any way by a 

 theory. Facts are sometimes awkward things — for theories. To 

 my mind there is nothing at all remarkable or improbable in Knappe's 

 statement: it would be a wonder — a miracle even — were the sup- 

 posed eggs in the testis to become actual functional eggs. They may 

 develop into spermatozoa, and they often do so, for they are the 

 forerunners of a second sort of spermatozoon. On the other hand, 

 there is nothing to prevent the occurrence of eggs and sperms in the 

 ovary. Without doubt the female can on occasion form spermatozoa 

 from the forerunners of male-eggs: the male can never differentiate 

 any sort of egg. Moreover, we know any number of instances of the 

 parthenogenetic development of eggs in the Metazoa, but not one of 

 that of spermatozoa. The female, therefore, "produces" two kinds of 

 gametes, either of which oq occasion being capable of developing with- 

 out the aid of another gamete or spermatozoon. On the other hand, 

 no instance of the parthenogenetic development of a spermatozoon is 

 known, or likely to be. The male only „produces" spermatozoa, often 



1) Fkiedmann, f., Rudimentäre Eier im Hoden von Rana viridis, 

 in: Arch, mikrosk. Anat., V. 52, 1898, p. 248—262. 



2) Knappe, E., Das BiDDEß'sche Organ, etc., in: Morph. Jahrb., 

 V. 11, 1886, p. 538—542. 



