OVUM. 



[83] 



Fig. 56*. 



Ovum, of the Rabbit in the tunica granulosa. 

 (.From Coste.) 



The middle part of one of M. Coste's figures has 

 been here copied to show the peculiar arrangement 

 of the granular cells round the disc and ovum, 

 which according to him are of the same nature 

 with those described by Dr. Martin Barry as 

 retinacula. It may be doubtful whether this 

 structure is constant. 



and bears no regular proportion in different 

 families of animals to the stature of the whole 

 body. In the mature state this variation ex- 

 tends from T o to 2^0 or g-i^ ". The following 

 are the results of a few measurements made by 

 myself and others of the external diameter of 

 the mature ovarian ovum, viz., man T -|^, dog 

 o, cat yi^, rabbit T ^, rat ^, mouse 



Pig ^o> cow ? i n , guinea-pig ^".^ 



The external tunic, or zona pellucida (a term 

 founded on the description of Von Baer*, from 

 its presenting the appearance of a transparent 

 ring between the opaque granular yolk-mass 

 within and the granular cells externally), is of 

 great proportional thickness ; viz., from T-gW 

 to 2sW /r > or from i to T \ of the whole 

 diameter of the ovum. When entirely freed 

 from the granular cells, its external surface 

 appears smooth ; and the inner surface, which 

 is exactly parallel to the outer, is also re- 

 markably smooth. The substance of this 

 tunic is very tough, and possesses considerable 

 elasticity ; so that the ovum and its zona may 

 be flattened by external pressure to a great 

 extent ; and yet it regains its nearly spherical 

 form when the pressure is removed. It is of 

 glassy transparency and homogeneous ; neither 

 any laminated, nor fibrous, nor other structure 

 being perceptible under the highest magnify- 

 ing power. It is easy to obtain evidence that 

 it is one thick membrane, and not composed 

 of two layers with intervening fluid, as some 

 have held, by cleaving it with fine needle- 

 points, when the cut edge becomes fully ap- 



* Epistola de Ovi Mammalium et Hominis genesi, 

 Lipsiae, 1827. 



parent. It has recently been stated by Remak*, 

 that in the mature ovarian ovum of the rabbit, 

 when freed from the granular cells, radiated 

 lines may be perceived running quite through 

 the zona pellucida ; these linear radiations, he 

 conceives, indicate a peculiar structure of the 

 zona, somewhat similar to the perforated con- 

 dition of the outer membrane of the ovum in 

 osseous fishes. I have perceived some of this 

 radiated appearance ; but I am inclined to be- 

 lieve that it depends not on any structure of 

 the zona itself, but rather on the marking 

 produced by the adhesion of parts of the cells 

 of the tunica granulosa, which become pedi- 

 culated in very ripe ova, and have then a ra- 

 diated appearance on the zona under pres- 

 sure ; as represented by Bischoff in his view 

 of the ovum of the dog, of which fig. 55 D. 

 is a copy. I shall have occasion afterwards 

 to state the nature of the fine canals which 

 have been observed in the outer tunic of 

 fishes' eggs. 



It has been customary among ovologists, 

 till very recently, to look upon the zona 

 as corresponding to the external membrane 

 of the yolk in the bird's egg. But from 

 what has been already said in connection 

 with that subject, the propriety of draw- 

 ing a distinction between these two enve- 

 lopes has been fully shown. I prefer, there- 

 fore, to retain the name of zona pellucida, 

 though it may not be perhaps the best de- 

 signation, as it prevents all confusion which 

 might be introduced by views of its analogies. 

 It will hereafter be more fully demonstrated 

 that it not only differs in its mode of origin 

 from the true vitelline membrane of birds, 

 but that it also has a different destination in 

 connection with embryonic development, in- 

 asmuch as, though formed in the ovary, it 

 remains and constitutes the basis of, or be- 

 comes incorporated with, the important struc- 

 ture which at a later period becomes the 

 chorion. 



In the foregoing description of the mem- 

 brane of the mammiferous ovum, I have 

 adopted the view first advocated by Coste-j-, 

 and by Thomas Wharton Jones J ; and more 

 fully brought out and established by the re- 

 searches of Bischoff in his admirable works on 

 the development of the Rabbit and the Dog. 

 It may be proper to remark farther, that though 

 the name of zona pellucida has been employed 

 to designate the thick single tough membrane 

 by which, as is now well ascertained, the yolk 

 of the mammiferous ovum is invariably enclosed 

 from an early period of its formation in the 

 ovary, this term is used synonymously with 

 that of vitelline membrane, and as applied to 



* Muller's Archiv. &c. for 1854, p. 252. 



f Recherches sur la Ge'ne'ration des Mammiferes, 

 4to. Paris, 1834, fig. 2.; and Embryoge'nie Com- 

 pared, torn. i. p. 200, Paris, 1837. 



I In a paper read to the Royal Society of London 

 in 1835 ; printed in the London Medical Gazette for 

 1838, p. 680. 



Entwickelungsgeschichte des Kanincheneies, 

 Braunschweig, 4to. 1842 ; and Entwickelungsg. 

 des Hundeies, Braunschweig, 4to. 1845. 

 [G 2] 



