290 LEUCKART, ON MICROPYJ.E. 



In general, he is of opinion that we are by no means justi- 

 fied in assuming the existence of a micropyle-apparatus 

 universally in the animal ovum. That for the purpose of 

 impregnation, it is in all cases necessary that the spermatic 

 filaments should come into immediate contact with the vitellus, 

 can no longer admit of doubt, from the result of recent ex- 

 periments, and particularly from the researches of Newport 

 (on the Frog's e^g), of Bisclioff and himself (on the Frog's and 

 mammalian ovum), of Meissner (on the mammalian ovum), 

 and of Lacaze Duthiers (on that of Dentalium), leaving out 

 of the question the observations on the eggs of Insects — but 

 this contact may probably be brought about in different 

 animals by different modes. In the same way that the 

 existence of an operculum, of valves, and similar provisions 

 for the liberation of the embryo from the coats of the ovum, 

 is limited only to certain species of animals ; that is to say, is 

 governed by certain external conditions, although the em- 

 bryos, without exception, are liberated : so, also, is it pro- 

 bable that the existence of a micropyle for the admission of 

 tlie spermatic filaments is confined within certain bounds. 

 We are already in a position partly to determine, a priori, the 

 conditions under which the presence of a micropyle in the 

 animal ovum is rendered a physiological necessity. This 

 will be the case especially in those instances in which the 

 ova are very early, and before they come in contact with the 

 spermatic fluid, surrounded with a firm and resistant en- 

 velope, the penetration of which would resist all the boring 

 powers of the spermatic filaments. This would happen more 

 especially with ova furnished with a chorion (that is to say, 

 with a second, usually very firm envelope, formed in the 

 ovary), in which we may predicate the existence of a micro- 

 pyle. To this kind of ova belong, also, nearly all those cases 

 in which we have hitherto found micropyle-organs — the ova of 

 Insects and of osseous Fishes — those of the Holothurice, and 

 also of the Bivalves. 



It can scarcely, perhaps, be assumed that our observations 

 respecting the occurrence of the micropyle in the ova of 

 animals are at present conclusive. We shall undoubtedly 

 meet with such a provision in numerous other animal forms. 

 In most cases, we should certainly not place too high a value 

 upon the negative results of earlier observations. Personal 

 experience will show how easy it is to overlook an apparatus 

 of the kind, especially when it is confined to a limited spot, 

 and is otherwise indistinctly indicated ; and it will be seen 

 that such a denial of its existence is unjustifiable. As 

 regards himself, Leuckart would remark that, notwithstanding 



