754 JOHN BEARD, 



IX. Concerning the Origin of Sex in tlie Metazoa. 



The sexual reproduction of the Metazoa is doubtless a modification 

 of an original isogamy. It would appear to be derivable from such 

 through a former heterogamy of twofold gametes, micro- and macro- 

 gametes. Perhaps it would be futile to speculate whether the actual 

 heterogamy of fourfold gametes arose prior to the formation of steril- 

 ised Metazoan persons. On this question the writer expresses no 

 opinion. One obvious result of the production of such an individual, 

 as an incident of the developmental cycle, has been to reduce, or to 

 keep short, the period of gametic union. 



As already elsewhere recorded, in the skate the union of the 

 nuclei of the two gametes is what may be termed a loose one; for, 

 exactly as already described by Rückert and Hacker in other and 

 similar cases, paternal and maternal parts of the nuclei remain distinct. 

 This is so for that portion of the cycle of the germ-cells, which cul- 

 minates in the formation of what have been termed the primary germ- 

 cells. That is to say, the duplication and loose union are retained, 

 until the germ-cells of like generation and equivalence with that which 

 formed the embryo, after the long resting phase — during which the 

 embryo is in course of evolution — divide and form secondary germ- 

 cells ; or, in other words, until the initiation of the ensuing sex-deter- 

 mination. 



We may, perhaps, look upon the reduction and sex-determination 

 as originally merely an undoing of the previous union, such that, if 

 the gametes were E and e, and S and s ^), where the two unions resulted 

 in ES and es as the zygotes, then these latter at the ensuing reduction 

 may have separated into E and a, e and s, but while E and s retained 

 their nature as female-egg and hair-like spermatozoon respectively, S 

 became the male-egg and e the second form of spermatozoon. Putting 

 the matter in tabular form, and assuming the continuity of the con- 

 jugating nuclei from generation to generation, we obtain the following 



result : 



~ ~ ' sperm 



1) E and e being the two forms of eggs, S and s the spermato- 

 zoa, respectively arranged and destined to unite with them. 



