50 



OVUM. 



the product of different successive years be 

 taken together, a fowl may, at the most, bring 

 forth about 1,200 eggs. I have attempted to 

 count and estimate the whole number of 

 ovula in the undeveloped state to be seen in 

 the ovary of the common fowl, and I find it to 

 amount to 30,000 or 40,000. A great many 

 of these ovula must, therefore, be unproduc- 

 tive in the higher oviparous animals, their 

 germs either remaining undeveloped or being 

 absorbed in the ovaries. 



In mammalia, and in the human species, 

 although only a few ovula approach to 

 maturity at a time, and a small number only 

 of the ova, as compared with the whole of 

 those contained in the ovaries, serve for pro- 

 duction of the offspring, it is known that a 

 considerable number is discharged from the 

 Graafian vesicles of the ovaries in the unim- 

 pregnated state. Thus, in the human female 

 (as will be more fully stated hereafter), one or 

 more ovula are discharged from the ovarian 

 vesicles at every successive four-weekly men- 

 strual period during about 30 years of life, 

 and thus not less than 400 ovula may be ex- 

 cluded from the ovaries ; but this number is 

 probably greatly below that of the whole 

 ovula or their germs, which the human ovaries 

 contain ; and the ovaries of many quadrupeds 

 present, undoubtedly, a greater number. 



External form and relation of the parts. 

 The ovum, being composed of cellular, gra- 

 nular, and fluid substance, and being enclosed 

 by an entire vesicular membrane, has gene- 

 rally, at least in its early condition, a spherical 

 form. In the mature state, this form is in 

 many instances retained ; but there is also not 

 unfrequently a departure from it, in conse- 

 quence of the addition of the external parts 

 unequally deposited on the surface of the 

 more globular yolk within them. This sphe- 

 rical form of the ovarian ovum points to its 

 isolated mode of production, and its destina- 

 tion for a separate existence, and is character- 

 istic of the elementary nature of its organic 

 structure. 



In the class of birds, the egg is always 

 covered in by an external hard calcareous 

 shell ; and in the greater number the external 

 form given by this is somewhat elongated, 

 and not unfrequently, as in the common fowl, 

 with a difference of the size and curvatures 

 at the opposite ends, caused by the manner 

 of the descent through the female passages. 

 There are, however, considerable differences 

 among the different families of birds, as in the 

 nearly globular form of the predaceous, the 

 more elongated form with nearly equal ends in 

 some of the ducks, the well-known shape of 

 the gallinae and others allied to them, and the 

 greater disparity at the opposite ends, as in 

 the seafowl.* 



[n those of the scaly reptiles which are 

 oviparous, there is also a firm external cover- 

 ing ; but only in some of them, as in most 

 chclonia and in the crocodiles, is there a hard 



* See on this subject the works of Hewetson and 

 others. 



calcareous shell. In the greater number of 

 the sauria and ophidia, the external covering 

 is of a tough membranous or parchment-like 

 consistence, formed of several layers of con- 

 densed fibro-alburninous substance, in which 

 either no calcareous matter, or only a small 

 quantity of it, is contained. In those serpents 

 and lizards, again, which are ovoviviparous, 

 the egg, when it descends into the oviduct, is 

 not covered there with the firm external en- 

 velope, but with a thinner and softer mem- 

 brane, similar somewhat to the membrane 

 lining the shell in birds. 



External forms of different eggs of Birds and 

 Reptiks. 



A. Batrachian reptile, frog or toad; spherical 

 shape, the dark yolk within, the gelatinous albumen 

 externally ; swollen by imbibition of water. 



B. Triton or Salamander; elongated external 

 membrane, coloured spherical yolk within. 



c. Oval of unequal curves at the two ends, as in 

 gallinaceous, passerine, and many other birds. 



D. Very unequal size of the two ends, as common 

 among sea-fowl. 



E. Equal oval, as among some ducks, the crocodile, 

 lizard, &c. 



F. Short oval or nearly spherical, as in predaceous 

 birds, chelonia, &c. 



In the oviparous cartilaginous fishes, a 

 peculiar horny capsule is formed round the 

 yolk and albumen, as they pass through the 

 oviduct at a place where a particular gland is 

 provided for its formation. These capsules 

 are of a fibrous structure, of an oblong, some- 

 what quadrilateral shape, as in the skate and 

 shark, and, at each angle, are prolonged into 

 tubular processes or filaments, of great length 

 in some sharks, which when short and open, 

 may allow of the passage of water to the 

 embryo contained for a long time within the 

 ovum; and serve also the purpose, when long 

 and convoluted, of entangling and attaching 

 the egg capsule among seaweed and floating 

 bodies. 



I have found that in the Myxine glutinosa 

 the globular yolk is enclosed in a horny cap- 



