[841 



OVUM. 



the only covering with which the ovarian ovum 

 is provided at the time of its leaving the Graa- 

 fian vesicle, excepting that which it retains 

 for a time derived from the cells of the tunica 

 granulosa in the proligerous disc. Von Baer 

 indeed, from whose description the term pellu- 

 cid area or zone has been borrowed, was not 

 full}' aware of its consisting only of one thick 

 membrane ; and more than once, both in the 

 Epistola and in the Commentary upon it, 

 expresses himself doubtfully as to whether 

 this pellucid space or halo* might not be 

 formed of an external envelope which he 

 terms the cortical membrane, and of another 

 situated within it. 



It is well known that when the mammi- 

 ferous ovum has been fecundated, and arrives 

 in the cavity of the uterus, and begins then 

 rapidly to expand, it is covered by a mem- 

 brane which soon undergoes great extension, 

 and acquires a villous structure on its external 

 surface, by which it may always be easily re- 

 cognised. This villous envelope of the ute- 

 rine ovum, universally now known as the. 

 chorion, is the product no doubt of changes 

 which only occur in their completeness after 

 fecundation, and as an accompaniment of 

 embryonic development ; but still as it ap- 

 pears probable that the zona pellncida is in- 

 timately connected with the first condition of 

 the chorion, and as the first formation of the 

 latter membrane is by many believed to take 

 place independently of fecundation or foetal 

 development, it is necessary for me to make 

 some remarks in this place on the relations 

 of the zona pellucida to the external cover- 

 ing which the ovum obtains in the first periods 

 of its residence within the female passages. 

 This is a subject on which embryological 

 writers are by no means agreed, and several 

 of them indeed have themselves changed their 

 opinions in regard to it in the progress of 

 their researches. 



Von Baer, the correctness of whose general 

 views on the phenomena of development we 

 have occasion to admire in almost every part 

 of the subject of which he has treated, was at 

 first of opinion that the chorion might arise 

 in Mammalia from the outer of the two layers 

 of which he at that time (though also doubt- 

 ingly) conceived the external covering of the 

 ovarian ovum to consist, and he looked upon 

 this as a great difference or departure from 

 analogy between the bird's egg and the ovum 

 of the mammifer: but in his work on Deve- 

 lopment f, published eight years later than the 

 Epistola and Commentary, he states his convic- 

 tion from his observations, that in some mam- 

 miferous animals at least, such as the pig and 

 sheep, the chorion is formed by external de- 

 posit on the surface of the ovarian ovum du- 

 ring its descent into the uterus, and therefore 

 takes its origin more in analogy with the ex- 



* Spatium pellucidum, halo vel peripheria lu- 

 cida. See Commentary on the Epistola, as trans- 

 lated by Breschet in his Repertoire, 1828, p. 52. 



t Beobachtunfi;. und Keflexion. liber Entwicke- 

 lunijsgescb. &c., Konigsberg, 1837, part ii. p. 185. 



ternal covering of the bird's egg. At the 

 same time he confesses that he was not able 

 to reconcile this view with what he had seen 

 in the dog and rabbit. 



Valentin, in his Manual of the History of 

 Development*, regarded it as most probable 

 that the chorion is formed by the deposition 

 and consolidation of an albuminous matter 

 on the surface of the ovum during its descent 

 through the first part of the Fallopian tubes ; 

 but though taking, as it appears, a perfectly 

 correct view of this subject, he did not bring 

 forward observations sufficient to establish 

 the opinion which he had founded chiefly on 

 analogical considerations. 



We owe to Thomas Wharton Jones the 

 first direct observation of the actual deposit 

 of a layer of albuminous matter round the 

 surface of the zona pellucida. But although 

 Mr. Jones, in the observation which he made 

 on the ovum of the rabbit within the Fallopian 

 tubes on the third day after conception, was 

 quite assured that a new structure had made 

 its appearance in considerable thickness on 

 the surface of the zona, yet he at first sup- 

 posed that this might proceed from some 

 change in the remains of the granular tunic 

 which adhered to that membrane after it had 

 left the Graafian follicle.f He afterwards, 

 however, became aware of the source of this 

 fallacy, and adopted the view that the mam- 

 miferous ovum receives a stiperadded struc- 

 ture, contributing to the formation of the 

 chorion, in its descent through the tubes. 



In the first series of his Embryological Re- 

 searches, Martin Barry described not only 

 the zona pellucida as recognised by other ob- 

 servers, but also a distinct vitelline membrane 

 within it; and he conceived that the zona 

 became afterwards the chorion. But in his 

 second series he became aware, both from the 

 statements of Wharton Jones and from his 

 own observations, that a new deposit occurs 

 in the rabbit's ovum ; and this new deposit 

 he now regarded as " the true chorion." He 

 retained, however, for a time his view of the 

 separate existence of a vitelline membrane ; 

 and thus described articulately three mem- 

 branes as belonging to the mammiferous ovum, 

 viz., vitelline membrane, zona peilucida, and 

 chorion. J The first membrane he believed 

 to disappear previous to the period of full ma- 

 turity : the two last he regarded as together 

 the source of the chorion of a later stage. 



Bischoft' also had not been aware from his 

 earliest observations that any new deposit 

 occurred round the ovum of the rabbit in the 

 tubes ; and even after he had become ac- 

 quainted with this fact, and had observed it 

 himself, as he did not detect a similar deposit 

 on the ovum of the dog, he adhered to the 



* Handhuch der Entwick. &c. des Menschen, 

 Berlin, 1835, pp. 38, 39. 



t On the First Changes in the Ova of theMam- 

 mifera, in consequence of Impregnation, and on the 

 Mode of Origin of the Chorion, in Phil. Trans. 

 1837, part. ii. p. 339. 



J See I'hil. Trans, for 1839, p. 316. 



