60 



OVUM. 



comparatively large veins which constitute 

 the hollow spaces. The ends of these veins, 

 then, look towards the inner surface of the 

 membrane ; and the appearance of a divided 

 cavity in some of the supposed follicles is 

 merely caused by two or more veins meet- 

 ing in a common dilatation at this place. 

 The capillary vessels, in passing into these 

 large commencements of the veins seem to 

 converge from its circumference to its centre. 



In the enlarged ovarian capsules of the 

 turtle, a somewhat similar arrangement may 

 be observed ; but I have not had an oppor- 

 tunity of tracing its relation to the blood- 

 vessels ; nor have I had the means of ascer- 

 taining whether anything of the same kind 

 exists in other reptiles with large yolks. In 

 the skate 1 have not been able to perceive 

 any similar arrangement ; and in the Graafian 

 vesicle of mammalia the lining membrane 

 presents internally a smooth surface destitute 

 of any appearance of depressions or of pecu- 

 liar venous sinuses. 



The appearance which I have just now 

 described had not escaped the notice of Von 

 Baer ; for at p. 23 of his work on develop- 

 ment, he mentions the existence of clearer 

 points in the inner membrane of the theca, 

 and states his opinion that they may be open 

 mouths of blood-vessels, by means of which 

 the yolk may be nourished by the direct 

 access of blood to it. 



In the naked amphibia and osseous fishes, 

 the ovaries (of which the general form has 

 been previously noticed) present a still greater 

 decrease in the proportion of the stroma to 

 the ovicapsules and ova. These capsules are 

 themselves also of much more delicate struc- 

 ture than in the higher animals ; but the rela- 

 tion of the ovules to the ovicapsules in their 

 formation, and the mode of their escape by the 

 rupture of the theca, are essentially analogous 

 to those of birds and reptiles. In the earliest 

 condition, it is true, the ovary may present 

 a greater amount of solidity in some of these 

 animals : but from the prodigious number of 

 the germs of the ovules and the small quan- 

 tity of the ovarian stroma, as soon as the 

 ovary has made some progress in develop- 

 ment, it acquires the appearance rather of 

 a mere mass of ova connected together by 

 a membrane and fine thread-like pedicles, 

 than of a solid or consistent organ containing 

 them. The delicate ovicapsules containing 

 the ovules embrace them closely as in the 

 large-yolked group of animals, there being 

 little or no fluid between the capsules and 

 the vitelline membrane. 



The structure of the ovaries in the inver- 

 tebrate animals presents so many varieties 

 that it would occupy too much space to 

 allude to them here. I refer the reader for 

 information regarding them to the article 

 ORGANS OF GENERATION, and others on par- 

 ticular classes and orders of animals in dif- 

 ferent parts of this work. For our present 

 purpose the structure of these organs has been 

 sufficiently indicated in the previous section. 



In conclusion, it may be right to recapitulate 

 the general nature of the ovary or formative 

 organ in its relation to the production of ova. 

 A comparison of the forms previously indicated 

 leads to the general view that the ovary is to 

 be regarded as analogous to the glandular 

 organs. _ In the great majority of animals 

 highest in the scale, the ovisacs are close fol- 

 licles from which the product of formation 

 (or secretion) escapes by the bursting of the 

 wall of the follicle in the highest animals, on 

 the external surface of the organ, in those 

 coming next in the series, towards an internal 

 cavity. In other instances, principally among 

 the lower animals, the structure is more ana- 

 logous to that we are accustomed to consider 

 as characteristic of the true glands, in which 

 the secreted cellular product is formed within 

 the same or a continuation of the tubular 

 ducts themselves by which they make their 

 escape. The more complex structure of the 

 capsules in which the large-yolked ovules are 

 produced in birds constitutes a special appa- 

 ratus, which, though without follicular com- 

 plication, may be looked upon as a modifica- 

 tion or higher degree of development of the 

 glandular structure of the ovary, provided 

 for the rapid formation of the larger mass of 

 nutritive substance which is present in these 

 ova. 



4. More detailed description of the ovum 

 of birds as the type of the 1st group. 



Having in the previous section given a 

 sketch of the general resemblances and dif- 

 ferences observed among the ova of various 

 animals, I now proceed to describe more in 

 detail an example from each of the three 

 groups previously distinguished, and more 

 particularly those of Birds and Mammalia, 

 which demand the greatest share of our atten- 

 tion in the study of development ; and first 

 as to the ovum of the common fowl. 



Quantity of matter, composition, $c. The 

 average dimensions of the fowl's egg in this 

 country are the following: The long diameter 

 2^ inches, short diameter If inch. The aver- 

 age weight of eggs of this size is a little more 

 than 2 oz. avoird., or 920 grains.* 



The extremes in weight which I have ob- 

 served among eggs of the fowl naturally formed 

 are 750 and 1060 grains. Double-yolked 

 eggs are, as might be expected, much larger, 

 reaching often a weight of 1400 grains, or 

 S^oz. 



The yolk weighs about a third of the 

 whole ; the albumen, membrane, and shell 

 forming the remaining two thirds. These 

 parts of the egg are in the following propor- 

 tions to each other in 100 parts ; the albu- 



* The following is a comparative view of the 

 average size and weight of the eggs of the com- 

 mon fowl, duck, turkey, and goose. 



Fowl - 

 Duck - 

 Turkey 

 Goose - 



Length Breadth 



(in inches). 

 2-25 1-7 



2-5 1-75 



2-7 1-9 



3-3 2-4 



Weight 



(in grains). 



920 nearly 2 oz. 



1100 2i oz. 



1.300 3 oz. 



2GOO 6 oz. 



