76 



OVUM. 



focus at the margin. E. The ovisac and ovum 

 more advanced ; o, v, ovisac, with epithelial lining ; 

 v, minutely granular yolk ; g, germinal vesicle. 



F. Part of an ovuie of ^ of an inch in diameter 

 highly magnified : v, minutely granular or primi- 

 tive yolk substance ; g, germinal vesicle ; 2, thick 

 consolidated membranous layer which formed a ve- 

 sicular covering for the primitive ovule, and which 

 corresponds to the zona pellucida of the niammi- 

 ferous ovule. 



i and K are intended to illustrate, diagrammati- 

 cally, the view, that after the disappearance of the 

 zona, and the formation of larger granular yolk cells, 

 the outer layer of the cells of this substance forms 

 the permanent vitelline membrane of the bird's 

 egg ; v, d, remains of minutely granular yolk, form- 

 ing the vitelline disc round the germinal vesicle ; 

 s,g, large corpuscles of the yolk ; v, m, outer layer of 

 the cells of the same, on which the vitelline mem- 

 brane is afterwards formed. 



been looked upon as the primitive or first- 

 formed structures connected with the origin 

 of the ova, the germinal vesicles subsequently 

 making their appearance within them. We 

 shall return to this point hereafter in con- 

 nection with the history of the mammiferous 

 ovum. My own observations agree with 

 those of Barry, as I have sometimes observed 

 very small germ-vesicles or cells in the ova- 

 rian stroina without any follicular covering. 

 But it must be admitted, at the same time, 

 that in birds the ovisac or ovarian vesicle is 

 formed so early that it is observed almost 

 always coexisting with the germinal vesicle or 

 rudiments of the ovule ; so that, if the latter 

 takes the precedence of the ovisac, it must be 

 by a very short period. 



According to Barry, there is seen almost 

 from the first, in the clear germinal vesicle, a 

 minute distinct granule or round spot, which 

 constitutes the first state of the macula germi- 

 nativa. Very soon the vesicle is surrounded 

 by a small quantity of a clear fluid in which 

 are rapidly deposited globules or granules 

 constituting the first rudiments of yolk 

 substance. There is no vitelline membrane, 

 however, in birds, at the first ; nor are the 

 larger cells which at a later period inter- 

 vene between the ovisac and the primitive 

 yolk, formed in the earliest stage. The smallest 

 ovisacs which Barry observed, and which con- 

 sisted of perfectly simple vesicular linings of the 

 cavities containing the rudimentary ova, in 

 the pigeon and common fowl, were from ^i n 

 to 300' f an nicn i' 1 diameter. * At a 

 somewhat later period, the number of maculae 

 (nuclei) in the vesicle, and of the yolk 

 granules externally, had increased, and a 

 delicate membrane, which he describes as 

 vitelline membrane, and believed apparently to 

 be the same which afterwards surrounds; the 

 large yolk in the fully-developed ovum, has 

 made its appearance. At this period also 



* Vide loc. cit. Plate v., figs. 18, 19, and 22 of 

 pigeon ; figs. 23 and 24 of common fowl. The mem- 

 brane which Barry described as vitelliue in the 

 earliest stages of growth of the bird's egg was pro- 

 bably not so, but the outline merely of the albumi- 

 nous substance in which the primitive yolk granules 

 are deposited. This will be made more apparent in 

 our description of the formation of the ova of 

 Batrachia. 



there begin to be formed within the ovisac a 

 se of larger nucleated corpuscles or cells, 

 which are external to the true ovum, and 

 which may be considered as corresponding 

 with the so-called granular contents (sub- 

 stantia and tunica granulosa) of the Graafian 

 follicle in mammalia. 



The early structure and development of 

 the ovum of birds have more recently been 

 described, with considerable detail, from ob- 

 servations on the chaffinch and common fowl 

 by Dr. H. Meckel* ; and as the observations of 

 this author have led him to take a somewhat 

 different view of the relations of some of the 

 parts of the ova of birds and other animals 

 from that which has hitherto been generally 

 adopted, it will be proper to give a particular 

 account of them in this place. Many phy- 

 siologists have felt the incongruity of the 

 comparison generally made between the mi- 

 nute and simple ovum of the mammifer, and 

 the large and more complex yolk of the 

 bird, and most are disposed to acknowledge 

 the necessity of making some more marked 

 distinction between the granular and the 

 cellular yolk substance in the two great 

 groups to which these ova respectively be- 

 long. It has before been stated, that Von 

 Baer on his discovery of the mammiferous 

 ovum, regarded it as corresponding, not to the 

 whole ovum of birds, but to the vesicle of Pur- 

 kinje. The discovery, in J834, of the germinal 

 vesicle in the mammiferous ovum, of the ex- 

 istence of which Von Baer had no distinct 

 knowledge, induced Valentin and others to 

 maintain that the essential parts of the ovum 

 are the same in the bird and the mammifer. 

 But it may be doubted whether physiologists 

 may not have proceeded further than they 

 were warranted by observation in regardingthe 

 vitelline membrane and large corpuscles of the 

 yellow yolk of birds as essentially corre- 

 sponding parts with the zona pellucida and 

 the smaller granular yolk of the mammifer. 

 For the membrana vitelli of the bird's egg 

 may, perhaps, be more analogous to the outer- 

 most layer of the membrana granulosa of the 

 Graafian follicle, and the large cellular yolk to 

 a part of the same substance or the fluid of 

 the Graafian follicle ; while the minutely gra- 

 nular yolk in which the cicatricula originates 

 and the germinal vesicle together are the true 

 representatives of the small ovum of the mam- 

 mifer. It seems undoubted, that what we term 

 the yolk membrane in the fowl's egg does not 

 exist in the early stages, and is formed indeed 

 only as the ovarian egg approaches maturity, 

 and it is admitted that no large cells similar 

 to those of the bird's yolk exist within the 

 cavity of the zona pellucida of the mammi- 

 ferous ovum. If this view is correct, we may 

 expect to find a representative in the egg of 

 the bird and of other animals having similar 

 ova, of the very marked enclosing vesicle, 

 which has received the name of zona pellucida 



* See his paper, Die Bildung der fur partielle 

 Fiirclmng bestimmten Eier der Vogel, &c., in Sie- 

 bold and Kiilliker's Zeitsch. fur Wissenschaft. Zool. 

 vol. iii. p. 420, 1852. 



