[132] 



OVUM. 



parent or female organ, in which the greater 

 part of the organised material first to be 

 employed in development is provided, and in 

 which the ovum arrives at a certain stage of 

 maturity ; and the other in its relation to fe- 

 cundation, or to the influence of the product 

 of the male by which its developing powers 

 are awakened or called forth. The mature 

 ovarian ovum may therefore, in one sense, be 

 looked upon as complete, if we regard only its 

 own structure ; but here its progress would 

 be arrested without the occurrence of fe- 

 cundation, and if we view it, therefore, with 

 reference to its more important destination as 

 the means of continuing the animal species, the 

 ovum can only be regarded as perfect when that 

 hitherto inscrutable change has been effected 

 on its substance by admixture with the minute 

 elements of the sperm in fecundation. The 

 constancy of this law in the whole animal king- 

 dom, with the exception of those of the Pro- 

 tozoa already referred to, makes it proper that 

 our definition should make reference to fecund- 

 ation as the means of perfecting the ovum. To 

 the nature of this process itself a further al- 

 lusion will hereafter be made. 



2. Recapitulation of the most general facts 

 ascertained by the comparison of the ova of 

 different animals. 



The ova of animals in their complete state 

 may be considered as consisting ot two sets 

 of parts which are of very different relative 

 importance in connection with the develop- 

 ment of the embryo : the first of these sets of 

 parts belong to the ovarian ovum, and are 

 formed previous to their quitting that organ ; 

 the others are subsequently formed, and may 

 be looked upon as accessory. These last often 

 present great varieties, so as to cause the ex- 

 ternal form and appearance of the ova of ani- 

 mals to differ widely, while the ovarian part 

 much more nearly corresponds. To this 

 ovarium ovum we shall principally confine our 

 present remarks. 



An extended comparison of the ovarian 

 ova of animals belonging to almost every 

 family of the animal kingdom has shown that, 

 notwithstanding great differences in size, and 

 some variation in form and structure, they all 

 agree in consisting of three essential and nearly 

 similar parts before the period of their detach- 

 ment from the ovary : these are, 1st, The in- 

 ternal nucleated cell or germinal vesicle with 

 its macula or maculae ; 2nd, The vitellus, or 

 yolk-substance ; and 3rd, The enclosing vesi- 

 cular envelope, or vitelline membrane. In all 

 animals there is, also, a general similarity in 

 the manner in which these parts are formed and 

 combined so as to constitute the ovarian ovum ; 

 the germinal vesicle is the first produced, and 

 may be regarded as the ovigerm ; the yolk- 

 substance next gradually envelopes it or is 

 deposited round the germinal vesicle, and in 

 general the vitelline membrane which encloses 

 the whole is the latest formed. 



The most marked differences among the 

 ova of animals are connected with the struc- 

 ture of the yolk and the relation which it bears 

 to the formation of the germinal part out of 



which the embryo is afterwards developed* 

 Founding upon this difference, three groups' 

 two principal and one subordinate, may be 

 distinguished among the ova of animals: 

 1st, The group of small-yolked ova, to which 

 belong those of Mammalia and a considerable 

 number of invertebrate animals, such as most 

 Mollusca, the lower Crustacea, most Anne- 

 lida, the Entozoa, Rotifera, Echinodermata, 

 Acalepha, and Polypina. In this group, the 

 ovum is generally of small or of moderate 

 size, as a whole ; the vitelline substance con- 

 sists entirely or chiefly of fluid with fine gra- 

 nular particles, and the entire yolk undergoes 

 segmentation : the entire yolk mass, therefore, 

 is directly formative, or is employed from the 

 first in the formation of the blastoderm or 

 organised substratum in which the embryo is 

 developed : the germinal vesicle is in this 

 group usually of small size, and has only a 

 single macula, or one composed of very few 

 particles. 



The second principal group comprehends 

 the large-yolked ova, such as those of Birds, 

 Scaly Reptiles, Cartilaginous Fishes, and the 

 Cephalopoda, to which, perhaps, may be added 

 Insects, Arachnida, and most Crustacea. In 

 this group, the largely developed yolk contains, 

 suspended in its basement, homogeneous sub- 

 stance, two kinds of organised corpuscles, viz., 

 1st, A certain portion of the small granular 

 part, similar to that of the small yolked ova, 

 which occupies a limited but determinate place 

 in the ovum, and in its centre the germinal ve- 

 sicle is situated ; and 2nd, A larger portion of 

 spherules, cell-like or other corpuscles of greater 

 magnitude. It is the first or finely granular 

 portion only which is immediately germinal, 

 or which is subject to segmentation and forms 

 the basis of the blastoderm ; the remainder, 

 or large cellular portion, is only secondarily 

 employed in supplying nourishment to the 

 embryo or its accompanying organised parts 

 in the progress of their development. In the 

 ova of this group, therefore, we distinguish 

 the formative or directly germinal portion of 

 the yolk-substance from the indirectly nutritive 

 portion. In these ova, the germinal vesicle is 

 also proportionally large, and the contents of 

 the vesicle, though consisting in the earliest 

 stages of their formation of a single macula, 

 or of a very small number, very soon become 

 converted into very numerous maculae, or into 

 a fine granular pulp. 



The third, or subordinate group, may com- 

 prehend the ova of Amphibia, or scaleless rep- 

 tiles, and osseous fishes, to which, perhaps, 

 may be added some of the invertebrate ani- 

 mals mentioned under the second group. The 

 ova of this group are intermediate in their 

 structure between those of the first and se- 

 cond : they have certainly the greatest affinity 

 with the large-yolked group, but there are 

 many gradations among the ova of this kind, 

 among allied species of animals, and it is chiefly 

 on the ground of the incompleteness of the 

 segmentation that I have thought it proper to 

 arrange them in a separate group. 



It may be remarked further, that in all ani- 



