MUSEUM OF COMPARATIVE ZOOLOGY. 113 



The development of a persistent egg membrane impervious to sperma 

 tozoa would evidently be possible only with the concomitant production 

 of one or more orifices; for without such provision no egg could be fer- 

 tilized, and the transmission of such tendencies would clearly be impos- 

 sible. That would necessitate the development of the micropyl© by 

 what I should call an exclusively progressive method. It would not 

 imply any regressive or resorbent process. How, then, could one find 

 reason for claiming any such process 1 I believe it would only be neces- 

 sary to assume that the zona radiata in its original development subserved 

 some useful end during the development of the ovum, in order to form an 

 idea of the possible course of events which has led to the present con- 

 dition. Imagine eggs at oviposition provided with a zona radiata which 

 remained or had become penetrable by spermatozoa; such eggs would be 

 in the most favorable condition for fertilization, but on account of the 

 condition of the zona they would be poorly protected against external 

 agencies. If, under these conditions, a portion of the zona radiata in 

 some eggs should become more resistant, even to the point of preventing 

 the entrance of spermatozoa, the eggs thus modified would be better 

 protected from injurious external influences than those which remained 

 in the original condition, and yet they would be almost as readily fertil- 

 ized as the latter, provided some portion of the zona remained, as at first, 

 penetrable. In short, the advantages of such a changed condition would 

 be greater than the disadvantages, and consequently in the long run the 

 more favorable condition would predominate. Evidently the optimum 

 protection would have been reached when a region no larger than that 

 absolutely necessary to admit a single spermatozoon, had been left for 

 that purpose. But this process of restriction in the area accessible to 

 the spermatozoa may easily have been accompanied by another process, 

 which may have begun as early as the former. The zona was assumed 

 to remain, or become at the time of oviposition, penetrable to spermatozoa. 

 It seems to me entirely reasonable that a process tending to modify a 

 portion of the zona and make it more readily penetrable should be set 

 up in the ovary, and that such eggs in the matter of fertilization alone 

 would have some slight advantage over eggs less easily penetrable. If 

 now these two tendencies were operative at the same time, — the one 

 serving to soften a part of the zona, in order to make it the more readily 

 penetrable, the other to harden another portion, to make the egg less 

 subject to adverse environment, — the former would become localized 

 by the encroachment of the latter until at length there would be only 

 a limited area in which the process of softening went on; but this might 

 vol. xix. — no. 1. 8 



