48 



OVUM. 



of late attracted considerable attention, and 

 appears to have been first clearly stated by 

 Ileichert in 1840, and afterwards in 1843*; 

 and in accordance with the views of that au- 

 thor, we may with propriety distinguish the 

 formative (or germinal) from the nutritive 

 parts of the yolk. In the fowl's egg, for ex- 

 ample (in which it must be admitted these 

 two parts were long confounded together), 

 the cicatricula, together with its so called 

 nucleus, and a part, perhaps, of the lighter- 

 coloured substance which occupies the centre 

 of the yolk and the canal extending from 

 it to the cicatricula, constitutes the formative 

 or germinal part ; and the larger mass of the 

 more deeply-coloured portion of the yolk 

 forms the nutritive vitelline substance. In 

 the mammiferous ovum, on the other hand, 

 the latter part is either entirely absent, or is 

 in small quantity, and the whole of the yolk 

 substance may be looked upon as directly 

 formative, or as analogous to that which forms 

 only the cicatricula of the fowl's egg. Among 

 more recent writers the distinction of these 

 parts has been particularly insisted upon, and 

 illustrated by M. Costef, and also by Messrs. 

 Prevost and Lebert.J 



The difference in the relative amount of 

 the formative and nutritive yolk substance, as 

 well as in the size of the whole ovum, in birds 

 and mammalia, is manifestly to be regarded as 

 more immediately connected with the dif- 

 ferent manner in which the embryo is to be 

 supplied with the materials necessary for its 

 growth in the two cases ; in the oviparous 

 mode of development, the whole amount of 

 nourishment required being provided in the 

 egg itself, and detached along with it from the 

 parent ; in the truly viviparous mode, a con- 

 tinual addition of new materials for growth, 

 being made by transmission from the maternal 

 parent in the placenta, or in some analogous 

 structure, which accompanies utero-gestation. 



The smaller proportional size of the nutri- 

 tive part in Batrachia and Osseous Fishes 

 (though most of these animals are truly ovi- 

 parous), may be attributed to the very early 

 period of development, and consequent small 

 size of the embryo at the time when in these 

 aquatic animals it leaves the egg, and, taking 

 upon itself an independent life, gathers nou- 

 rishment in the same manner as the adult 

 animal. $ 



* Entwickelungsleben im Wirbelthierreich ; 4to. 

 Berlin, 1840 ; and in Beitriige zur Kcuntniss des 

 heutigen Entwickelungsgeschichte; 8vo. Berlin, 

 1843, p. 22. 



t Cours d'Embryoge'nie Comparc'e, torn. i. Paris, 

 1837 ; and Histoire gen. et partic. du Developpement 

 de 1'Homme et des Animaux, Paris, torn. i. 4to., 

 1848. 



J In Annal. des Scien. Nat. for 1844. 3rd Ser. 

 torn. i. p. 193 and 265. 



At the same time it is to be kept in mind that 

 there are exceptions to these relations, which make 

 it extremely difficult to state any general law of 

 connection between the structure of the ovum and 

 the mode of gestation and place of development of 

 the embryo ; as in the case of a few of the lizards 

 and serpents, and some cartilaginous fishes, in which 

 although the egg agrees in general structure with 



The above arrangement is by no means 

 offered as exhausting the divisions which 

 might be formed of the ova of animals, but 

 rattier as bringing forward prominently the 

 most remarkable characteristics of those of 

 vertebrata. It is not improbable that a more 

 accurate acquaintance with the structure of the 

 ova in the animals thus grouped, and more 

 especially of the Invertebrata, may lead to some 

 considerable modifications of the divisions here 

 adopted; but the main distinction upon which 

 they are founded is so important, that even 

 with our present incomplete acquaintance with 

 them, it seems advisable to call attention to 

 it at this place. As I shall have occasion 

 to refer frequently to these groups in the sub- 

 sequent description of the ova of various 

 animals, in the absence of more appropriate 

 appellations, I will, for the sake of brevity, 

 designate them severally, as follows, viz., 1st 

 group, Small-yolkedova,asin Mammalia; 2nd 

 group, Large-yolked ova, as in Birds, Scaly 

 Reptiles, and Cartilaginous Fishes; 3rd group, 

 Middle-sized yolked ova, as in Batrachia, Os- 

 seous Fishes, &c. 



2. Further comparison of the ova of animals 

 in general, as respects their size, number, form, 

 and the relation of their parts. 



Size of ova. In addition to what has been 

 said on this subject in the previous section, it 

 may farther be remarked that, in the second 

 and third groups, the size of the ova of dif- 

 ferent genera and species is to a certain extent 

 proportional to that of the adult animal, or of 

 the fully-developed foetus; but in the first 

 group, or at least in Mammalia, in which the 

 nutritive part of the yolk may be considered 

 as wholly or nearly entirely absent, there is a 

 much greater uniformity in the size of the 

 ova; and, accordingly, the largest mammi- 

 ferous animal may take origin from an ovum 

 which, when mature, is even smaller than 

 those of species of animals many hundred 

 times less in bulk ; while in the class of 

 Birds we observe the nearly regular increase 

 of the size of the ovum in proportion to that 

 of the parent animal, from the smallest hum- 

 ming bird up to the ostrich, or the still larger 

 egg of the ^Epyornis, an extinct bird, of which 

 some of the bones, along with the eggs, have 

 recently been discovered in Madagascar.* 



that of animals which are generally oviparous, it is 

 retained in the oviduct or uterus of the female during 

 a part or even the whole of the time of fcetal develop- 

 ment; and there are also exceptions in the third 

 group viz. that of batrachia and osseous fishes as 

 in the Land Salamander and Viviparous Blennv. 

 To this mode of gestation the name of Ovoviviparous 

 has been given. There are many varieties of a 

 similar kind among the Invertebrata, and on the 

 whole it may be stated that there is no constant 

 correspondence between the size of the ovum and 

 the mode of gestation. The Marsupiata also, and 

 the Monotremata among the Mammalia, exhibit 

 interesting modifications, in the iirst a partial, and 

 in the second, probably a complete residence of the 

 ovum in the uterus of the female parent during de- 

 velopment ; while the ovum in these animals ap- 

 proaches, in some respects, the type which is more 

 commonly oviparous. 



* The circumference of this extraordinary egg 



