78 



OVUM. 



about -sV or ^ of an inch in diameter, by the 

 clearing and partial consolidation of the 

 outermost part of the albuminous basis or 

 blastema in which the granules of the pri- 

 mitive yolk substance are deposited. It is 

 at first comparatively thin : it is most ap- 

 parent by its greater clearness and consistence 

 in ovules of -^ or A of an inch in diameter, 

 in none of which have I ever failed to observe 

 it. In those of JQ ^ becomes broader, but less 

 clear and somewhat softer in its consistence, 

 and is uneven and as if softening away or 

 breaking up on the external edge ; and in 

 ovules of T V of an inch it has in general dis- 

 appeared. At no period have I observed it 

 to assume the glassy transparency, nor has it 

 the distinct outline and membranous appear- 

 ance represented by H. Meckel ; but it seemed 

 rather like a portion of the albuminous basis 

 of the yolk substance, nearly but not quite 

 deprived of the granules, which are thickly 

 deposited in the rest. 



While these differences are stated, how- 

 ever, it appears to me warrantable to coincide 

 in so far with the view of H. Meckel as to 

 regard this structure as a temporary repre- 

 sentation in the fowl's egg of the zona pel- 

 lucida, which in the mammiferous ovum 

 assumes greater consistence, passes into the 

 membranous form, and constitutes the only 

 ovarium covering of the ovule. 



In ovules of -fa to J^ f an inc h in dia- 

 meter, the layer of nucleated cells placed be- 

 tween the primitive ovule and the membrane 

 of the follicle, and which may be looked upon 

 either as a cellular lining of the follicle or 

 a peculiar investment of the ovule, has be- 

 come more distinct and consistent, and the 

 cells of the outermost layer have assumed 

 the form of short compressed prisms. They 

 have finely granular contents and clear 

 nuclei, with one or sometimes two nucleoli, 

 like the cells of the tunica granulosa of the 

 mammalian follicle. It appears that these 

 cells come afterwards to form the external 

 part of the yolk of the bird's ovum, the cel- 

 lular part of the yolk being formed within 

 them, and the vitelline membrane being pro- 

 duced on their outer surface. 



The bird's ovule, it is well known, usually 

 fills completely the ovarian follicle ; but in 

 several instances I have observed, from im- 

 bibition of water or some other cause, the 

 ovule to occupy not more than half the dia- 

 meter of the follicle, the remainder being 

 filled with a clear fluid ; and in these in- 

 stances the prismatic layer of cells adhered 

 closely to the surface of the zona and primi- 

 tive granular yolk. 



In ovules of from JQ to TV f an mc h tne 

 formation of the vitelline membrane appears 

 to commence. The external nucleated cell 

 covering has increased in quantity, and ad- 

 heres more closely to the granular yolk, with 

 which it generally comes away when the 

 follicle is opened. The external edge of the 

 layer of prismatic cells, the length of which 

 is considerably increased, is now surrounded 

 by a narrow pellucid space enclosed by a 

 double line, presenting the appearance as if a 



small part of the bases of these cells had 

 been fused together in a homogenous film. 

 This is the commencement of the formation 

 of the true vitelline membrane, which in the 

 latest period of ovarian growth of the ovum 

 becomes nearly quite structureless, but which 

 throughout the greater part of the process 

 retains somewhat of the hexagonal marking 

 from the close adhesion of the cells by the 

 amalgamation of a part of which the mem- 

 brane has been produced. 



In follicles of about -^ of an inch or larger 

 the true cellular elements of the yolk have 

 begun to be formed. The manner in which 

 these originate I have not as yet had the 

 means of determining with precision. They 

 appear, as H. Meckel has suggested, to be 

 produced as a secretion from the interior of 

 the ovarian follicle ; but they are more im- 

 mediately formed within the layer of pris- 

 matic cells which envelope the whole ovule. 

 Nor have I been able to determine their 

 precise relation to the zona and primitive 

 fine granular yolk. When the zona has dis- 

 appeared, as is usually the case in ovules 

 of -JQ- of an inch, the cellular elements of the 

 yolk seem to begin to mingle with the finely 

 granular substance. At first, cells and larger 

 oil globules, similar to those of the yolk 

 cavity, are produced. So long as these alone 

 exist, the yolks have a dull milky-white as- 

 pect. Later, or in those of of an inch, 

 or even somewhat smaller, the yellow tinge 

 appears, and this very soon becomes more 

 decided, at the same time that the peculiar 

 large granular corpuscles or cells are formed 

 in which the vitelline colouring matter prin- 

 cipally resides. These corpuscles are pro- 

 duced in successive layers within the layer 

 of prismatic cells of the external tunic ; and 

 it is probable that there is some periodical 

 variation in the rapidity of their formation 

 from the alternation of concentric layers of 

 deep yellow corpuscles with those of a lighter 

 colour which may be observed in the section 

 of a boiled yolk. The corpuscles of the 

 yellow yolk are not, however, formed in equal 

 quantity in every part : indeed they are quite 

 deficient at that side on which the remains 

 of the primitive granular yolk with the im- 

 bedded germinal vesicle are situated. 



With regard to the germinal vesicle, it is 

 to be observed that it is at first quite free in 

 the more fluid internal part of the primitive 

 yolk ; in the next stage, before the formation 

 of the cellular yolk, the germinal vesicle 

 comes to be imbedded in the more consistent 

 external part of the primitive granular yolk ; 

 and it is, no doubt, chiefly the remains of this 

 which form the germinal or vitelline disc : but 

 further observations are still required to de- 

 termine the precise manner in which this ar- 

 rangement of the primitive yolk in the discoid 

 shape takes place. 



It appears, further, that in the formation 

 of the elements of the more advanced yolk, 

 the earliest produced, or innermost, remain 

 the softest ; the latest formed, or outermost, 

 acquire the greatest consistence. The ex- 

 ternal layer of prismatic cells next the vitel- 



