STKUCTUKE OF OVARIAN O.VUM 



FIG. 16. OVARIAN OVUM OF A MAMMIFEB. 

 (Allen Thomson.) 



a, the entire ovum, viewed under pressure ; the 

 granular cells have been removed from the outer 

 surface, the germinal vesicle is seen in the yolk- 

 substance within ; b, the external coat or zona, 

 burst by increased pressure, the yolk-protoplasm 

 and the germinal vesicle having escaped from 

 within ; c, germinal vesicle more freed from the 

 yolk-substance. In all of them the macula is 

 seen. 



to an investment of several layers of epithelial cells derived from the 

 proligerus of the Graafian follicle (fig. 15). In the ripening oocytes these follicular 

 cells form a syncytial layer immediately applied to the zona. Within the zona a 

 second membrane of great tenuity has 

 been described by various authors 

 (recently by Van der Stricht in the 

 human ovum), but its existence has 

 been denied by many, and it is 

 supposed by some that the processes 

 of the follicular cells are continued 

 through the zona into the egg-proto- 

 plasm. The fact that when the zona 

 is ruptured in young oocytes the con- 

 tents do not separate from it is in 

 favour of such connexion (Ebner). 

 In young oocytes there does not seem 

 to be any space round the protoplasm. 

 The egg-protoplasm is 'filled with 

 globules and granules of different sizes, 

 but all possessing a high index of re- 

 fraction. Compared with that of some 

 lower mammals, the human ovum is 

 distinguished by the ill- defined cha- 

 racters of its specially minute deuto- 



plasmic bodies. When examined in the fresh condition the egg (fig. 15) is very 

 transparent, the deutoplasm being massed towards the centre, while there is a 

 clear or very finely granular layer round the periphery. 



The deutoplasmic bodies in the egg-protoplasm are spoken of collectively as the yolk. There 

 is great variety in the different orders of animals in respect of the amount of yolk stored in the 

 egg. The term aleciihal is used to denote an ovum such as that of the mammal, Amphioxus, and 

 many echinoderms, in which the yolk-particles are absent, or very small in amount and uniformly 

 distributed, while a heavily yolked egg in which the yolk is accumulated at one end is termed 

 tdolecithal. There are many different degrees of this condition (amphibians, fishes, reptiles, and 

 birds). In the very exaggeratedly telolecithal egg of the bird the amount of yolk- substance 

 is so vastly greater than that of the protoplasm itself thaf it is only in the neighbourhood 

 of the nucleus (germinal vesicle) itself that the protoplasm can be distinctly recognised. 

 This point is conveniently distinguished as the animal pole, as opposed to the vegetative pole 

 marked by accumulation of yolk-material. 



The amount and distribution of yolk is the main factor in determining the character of 

 the egg-cleavage. It is related to the determination of the period when the organism will 

 assume an independent existence. Thus in the bird the ovum contains necessarily all the 

 nutriment required by the chick until it is sufficiently developed to emerge from the egg and 

 obtain food independently. In the frog the yolk is sufficient for the early stages only and the 

 tadpole is hatched in a very immature condition, while in Amphioxus and many invertebrates 

 the organism is set free still earlier as a free-swimming ciliated blastula. 



In the mammal the conditions are wholly different, and the small amount of nutritive material 

 in the egg is obviously related to its retention in the uterus, from which it is able directly to derive 

 its nutriment. It is probable that the alecithal condition of the mammalian egg is a secondary 

 condition related to the introduction of uterine gestation. 



Imbedded in the protoplasm, usually eccentrically, is a large spherical nucleus, 

 which was termed by its discoverer, Purkinje, the germinal vesicle. 2 This, which 

 is about 30 to 45 ^ in diameter, has all the characters of the nucleus of a 



1 Bull, de 1'Acad. Roy. de Medicine de Belgique (4th ser.), xix. 1905. 



2 Purkinje discovered the germinal vesicle in the bird's ovum in 1825 ; that of mammals was first 

 noticed by Coste in 1883. 



