114 BULLETIN OF THE 



be — as we must believe often happens in the animal economy — corre- 

 spondingly intensified, until an activity which in the beginning resulted 

 in only a feeble modification in the condition of the zona ultimately ter- 

 minated in its complete liquefaction and absorption. 



A single argument, which it seems to me may have some value, in 

 support of this hypothesis, is to be drawn from the condition of the zona 

 in Elasmobranch fishes. So far as I now see, the complete disappear- 

 ance of the zona at the maturity of the egg would be entirely in har- 

 mony with the hypothesis. The condition there at any rate seems to 

 me to favor the assumption previously made, that the zona originally had 

 a function distinct from that which it now appears to possess in protect- 

 ing the embryo after fertilization. For if not, it must be an inheritance 

 from ancestors which, like bony fishes, had their embryos thus protected. 

 There are probably few who would defend the idea that the Elasmo- 

 branchs are descended from bony fishes, and the evidence of common 

 ancestors with eggs thus protected still remains to be found. 



If, as the Eussian naturalists assert, there are several micropylar open- 

 ings in the egg-shell of sturgeons, it may be that those fishes present a 

 condition which is intermediate between an extensive region of penetra- 

 bility and the extreme restriction which now prevails in bony fishes. 



The funnel portion of the micropylar region is certainly the less essen- 

 tial and least constant part of the structure. It may reasonably be 

 considered, I believe, a secondary condition, and the explanation of its 

 development might lie either in the fact that it served, in a passive way, 

 to direct the motion of a greater number of spermatozoa toward the 

 actual orifice, or, possibly, that it served to preserve from accidental 

 removal the protective products of the degenerated micropylar cell. 



Thus the micropylar canal might be regarded as the result of two to 

 a certain extent antagonistic tendencies, — the fittest solution of a prob- 

 lem requiring the fulfilment of two conditions. The micropylar funnel 

 could obviously be regarded as a partial compensation for the diminished 

 opportunities for fertilization caused by a restriction of the area available 

 for that purpose, and might have arisen simultaneously with the restric- 

 tion, or only after the latter had attained its present maximum. 



