[108] 



OVUM. 



pally its outer part. The vitelline membrane 

 does not exist at first, but seems to be formed 

 at a later period by the consolidation of an 

 external layer of the primitive yolk substance. 



The time of the disappearance of the ger- 

 minal vesicle has not been determined in 

 many of these Mollusca. Previous to seg- 

 mentation a phenomenon occurs, which has 

 now been observed in a large number of ani- 

 mals, but which first attracted special atten- 

 tion in the gasteropodous Mollusca; viz., the 

 separation of one or more clear hyaline liquid 

 globules of considerable size from the surface 

 of the yolk substance, into the space be- 

 tween it and the vitelline membrane. This 

 was first observed by Dumortier*, and de- 

 scribed by Pouchetf, by Van Beneden in 

 the Aplysia J, by Nordmann in Tergipes Ed- 

 wardsii $, by C. Vogt in Actoeon || , and by 

 various others. A precisely similar phenome- 

 non has also been observed in some of the 

 Vertebrata, as in Mammalia by Wharton 

 Jones, Barry, and Bischoff, and in Batrachia 

 by Newport. But though this separation of 

 one or more hyaline globules from the yolk- 

 substance at the time of segmentation appears 

 to be a very general accompaniment of that 

 process, it must be confessed that its import, 

 either in connection with fecundation or de- 

 velopment, has not yet been ascertained. 



Acephala. In Acephalous Mollusca the 

 ova are generally of small size, the yolk-sub- 

 stance principally finely granular, the germinal 

 vesicle clear, with a distinct macula, which last 

 not unfrequently presents the form of a double 

 or elongated biscuit-shaped particle. The 

 vitelline membrane is distinct and possesses 

 considerable strength ; and there is generally 

 a considerable space occupied by clear fluid 

 between it and the surface of the yolk. The 

 most interesting feature of the ova of these 

 Mollusca is the funnel-shaped aperture 

 which most of them possess, leading through 

 the vitelline or external membrane into the 

 space occupied by the yolk. This aperture, 

 styled micropyle by J. Miiller in the Ho- 

 lothuria, the first instance in which it was 

 discovered, in 18591F, was observed in the 

 ova of Unio and Anodonta by Leuckart ** 

 and Keber. ff The latter author supposed 

 that he had observed the penetration of a 

 spermatozoon into the ovum through this 

 aperture, and has described with great form- 

 ality and minuteness all the phenomena which 

 he conceived were related to that process. 

 Although Keber was correct in asserting the 

 existence of the micropyle in these Mollusks, 



* Embryol. des Mollusques, in Annal. des Scien. 

 Nat. for 1837, p. 136. 



f Id. lib. for 1838, vol. x. p. 63. See also Pou- 

 chet's further observations in his work, Theorie 

 positive de 1'Ovulation spontanee, pi. xvi. 



J Annal. des Scien. Nat. 1841, p. 126. 



Id. lib. 1846, p. 147. 



|| Sur PEmbryol. des Mollusques Gasteropodes, id. 

 lib. 1846, p. 33. 



f Archiv. 1852, p. 19. 



"* Article Zeugung, p. 801, Ann. 1853. 



ft De Introitu spermatozoorum in ovula, &c. 

 Konigsberg, 4to. 1853. 



Fig. 112 



Ova of Unio in different stages of development. 



(A. B. C. and D., from Hassling ; E. from Keber.} 



A. The early stage of the ovum, when the ger- 

 minal vesicle alone is distinguishable lying in a 

 bulging part of the ovarian substance. 



B. The same somewhat more advanced; the 

 ovicapsule and vitelline membrane have assumed 

 the pediculated form, and the yolk granules sur- 

 round the germinal vesicle. 



c. The ovum now enlarged and spherical in form, 

 the yolk granules increased in quantity, and the 

 pedicle narrowed so as to form a short micropyle 

 tube ; s, the small body taken by Keber for a sper- 

 matozoon, existing long previous to the occurrence 

 of fecundation. 



D. The ovum, &c. at a later stage ; g, the ger- 

 minal vesicle ; v, the yolk ; t/, the separated portion 

 of the yolk ; s, as in c. now enlarged. 



E. A nearly similar stage of the ovum as figured 

 by Keber. Some of the contents of the separated 

 portion of the yolk are escaping through the micro- 

 pyle aperture ; s, Keber's alleged spermatozoon. 



it appears that the body described by him as 

 spermatozoon cannot have been of that nature, 

 seeing that it has been proved by other 

 observers that the appearance on which 

 Keber's supposition was founded existed 

 long before fecundation, and remained 

 long after the commencement of embryonic 

 formation in the same condition.* 



The existence of a similar aperture or micro- 

 pyle in several other Acephalous Mollusca 

 has been ascertained by the recent investiga- 

 tions of various authors ; but the actual en- 

 trance of the spermatozoa by the aperture, 

 has not, so far as I aware, been satisfactorily 

 observed. There seem, however, to be suffi- 

 cient grounds for believing that in the Ace- 

 phala, as in other animals in which it is found, 

 the micropyle is immediately related to the 

 process of impregnation, by affording a ready 

 access of the spermatozoa to the yolk through 

 the more resistent membranes of the ovum. 

 The accompanying figures from Keber and his 

 critic Hessling give a sufficiently clear view of 



* Hessling, in Zeitsch. fiirWissensch. Zool., 1854, 

 vol. v. p. 380. ; and Bischoff, "Wiederlegung des von 

 Dr. Keber bei den Naiaden, &c. Giessen, 4to. 1854. 



