OVUM. 



75 



half of the diameter of the whole ovule. In 

 the progress of growth, it enlarges some- 

 what, but only in the earlier periods, and in 

 less proportion than the yolk, and undergoes 

 no farther increase during the greater part of 

 the time that the yolk acquires the greatest 

 addition of new matter. It is worthy of 

 remark, however, that the germinal vesicle is 

 originally of a large size in the eggs of birds 

 and other large-yolked ova ; that it is also of 

 very considerable size, even proportionally 

 larger, in the batrachia ; and that in mammalia, 

 and other animals with the smaller and gra- 

 nular yolk, its size bears in general a propor- 

 tion to that of the yolk. 



The substance of the yolk appears, in the 

 first place, to be simply granular, or to be 

 composed entirely of minute molecules such 

 as those which always form the yolk in mam- 

 malia. These are united together by a some- 

 what glairy fluid ; larger spherules gradually 

 appear among them ; and next the distinction 

 between the substance of the proligerous disc 

 and of the yolk cavity becomes apparent. 

 Lastly, 'the deep-coloured yolk corpuscles 

 are produced, layer after layer being deposited 

 from the exterior, so that the outermost are 

 the last formed. Externally a closer-set layer 

 of nucleated cells covers the surface, in con- 

 nection with which the vitelline membrane is 

 formed. 



The vitelline membrane is not formed at 

 an early period in the bird's egg : it cannot 

 indeed be perceived in ovula of a tenth of 

 an inch in diameter. We shall presently see 

 that its relations and mode of formation are 

 peculiar in the bird's egg. 



Morphology of the bird's egg as ascertained 

 from its first origin and development. The 

 ovaries of the common fowl, and indeed of 

 most large birds, are less favourable for the 

 investigation of the first origin and earliest 

 condition of the ovule, than those of the 

 smaller tribes ; this arises, not so much 

 from the dense structure of the ovary in the 

 undeveloped state, as from the great opa- 

 city produced in the ovules themselves, almost 

 from the first, by the deposit of thick-set 

 yolk granules. In some of the smaller singing 

 birds, the thrush, yellow-hammer, or chaf- 

 finch, the parts are clearer and more trans- 

 parent ; and it will be found that the pheno- 

 mena of earliest formation are most easily 

 investigated in them. 



According to Dr. Martin Barry's observa- 

 tions, in birds as well as in other animals, the 

 germinal vesicle is the part of the ovum which 

 is first formed. In the pigeon and common 

 fowl, he has observed these vesicles in the ova- 

 rian substance at a very early period*; and he 

 believes their origin as simple cells to precede 

 that of the ovarian vesicles, or follicles, or, as 

 he has termed them, ovisacs, which surround 

 them at a somewhat later period, but still in 

 the earliest stages of the formative process. 

 By other observers the ovarian vesicles have 



* See Philos. Trans, for 1838, p. 309. In this, 

 coinciding with the opinion previously expressed by 

 Von Baer. 



Fig. 53. 



B 



Earliest stages of the formation of the ovarian 

 eqg in the Bird. 



A, B, c, D, E, F, actual representations of portions of 

 the ovarian stroma and ovisacs of the thrush ; 

 G, n, i, K, diagrammatic sections of the same. A. In 

 the ovarian stroma are seen the earliest state of the 

 ova and ovisacs that can be perceived, consisting, 

 first, of minute granular spots ; next, of clear points 

 within a minute granular mass ; and third, of small 

 germinal vesicles, surrounded with the minutely 

 granular dark yolk substance. Compare with G, in 

 the diagrammatic figure. 



B and c. Different stages of formation of the ovi- 

 sac round the small ova : the epithelium is seen to 

 line the sac : the germinal vesicle with occasionally 

 a single macula is now apparent. D. The epithe- 

 lium of the ovisac shown iu focus over the whole 

 surface : in the other figures it is only shown in 



