70 SURVEY OY THE REPRODUCTIVE PROCESS 



stalk — as in those of Ecliinodermata and Helmintha — the 

 portion of the hollow pedicle which remains in connec- 

 tion with the ovum becomes the micropyle, but in 

 other cases a special aperture appears to be formed for the 

 purpose, in the previously continuous wall of the ovum. A 

 micropyle has now been observed in the ova of all insects, 

 and in those of Acephalous Mollusca, and of Osseous Fishes, 

 as well as in those of some Crustacea, Annelida, Echinoder- 

 mata, and Nematoid Entozoa. In the case of the Cepha- 

 lopoda and Batrachia the evidence is less satisfactory, and 

 no trace of it has yet been detected in the Mammalian 

 ovum, though the preseiice of spermatozoa in its interior 

 has now been verified by more than one observer. Here, 

 therefore, we can only conjecture that they may gain access 

 by a sort of boring action, or by the formation of extempo- 

 raneous apertures.* The micropyle is certainly not the 

 only provision for the purpose. In some cases the proper 

 wail of the ovum disappears before it comes into connection 

 with the Spermatozoa, which then penetrate into it over its 

 whole extent, their filamentous extremities giving the sur- 

 face a ciliated appearance. This has been observed by 

 Meisner in the Lumbricus, and may perhaps occur also in 

 some Hirudinei and Mollusca. In the Helmintha ao-ain, 

 the spermatozoa are frequently brought in contact with the 

 ova while the latter are yet in process of formation, as will 

 be more fully mentioned in the notice of that class. In the 

 Hydrozoa, too, impregnation seems to be effected in the 

 nascent condition, as it were, of the ova, for these bodies 

 do not appear to have any proper investing membrane, 



* In tlie ovum of the rabbit Dr. M. Barry describes (Op. Cit.) the for- 

 mation of a cbiak for the entrance of the Spermatozoa. In that of the 

 frog, according to Mr Newport, there is no perforation, aperture, or 

 fissure of any kind visible to the eye, but he has distinctly observed the 

 penetration of the Spermatozoa in considerable numbers and with great 

 rapidity (in less than a minute). Philos. Transactions. 1853. 



