OVUM. 



43 



PART SECOND. 



OF THE OVUM PREVIOUS TO THE COM- 

 MENCEMENT OF FOZTAL DEVELOPMENT. 



I. ANATOMICAL STRUCTURE, CHEMICAL 

 COMPOSITION, ORIGIN AND FORMATION OF 

 THE OVUM IN MAN AND ANIMALS. 



I. Preliminary and General Comparison 

 of the Ova of Animals. 



At a time when the analogy and difference 

 in the structure and functions of the ova 

 of various animals were less known than at 

 present, and especially before the discovery 

 of the true mammiferous ovum, the pheno- 

 mena of development were almost exclusively 

 observed in the eggs of birds ; and conse- 

 quently in the progress of investigations on 

 this subject, and their extension to other 

 animals, the nomenclature of parts of the egg, 

 and 'the interpretation of the phenomena of 

 development, came naturally to be founded on 

 those which had previously been adopted in 

 the study of the egg of the common fowl. In 

 more recent times it has been found that the 

 egg of birds presents in some measure excep- 

 tional characters, as compared with that of the 

 greater number of animals ; and it has thus 

 become apparent that the close limitation of 

 our observations to the class of birds, fruitful 

 as they are universally acknowledged to have 

 been in most important and interesting in- 

 formation on our subject, might tend even to 

 retard in some degree the establishment of the 

 more general laws of typical structure and 

 formative change, which constitute so remark- 

 able a feature of the result of modem em- 

 bryological researches. 



Still, the convenience of being able to ob- 

 tain the egg of the bird without trouble in all 

 stages of advancement, the familiar knowledge 

 that has so long been possessed of the pheno- 

 mena of incubation, the proportionally large 

 size of the embryo, the difficulty, on the 

 other hand, of making serial examinations 

 of the ova of mammalia, and, though nume- 

 rous and important, the comparatively frag- 

 mentary nature of the observations in this 

 class, and especially in the human subject, 

 make it desirable that we should not too soon 

 depart from the practice, which has so long 

 prevailed, <?f making the bird's egg the main 

 foundation of our description of the ovum and 

 formative process in general. But, at the 

 same time, the exceptional structure of the 

 fowl's egg alluded to renders it proper for me 

 to present here, in a connected form, some 

 account of the principal differences among the 

 ova of various animals ; so that we may, in 

 some measure, avoid drawing false analogies, 

 or deducing supposed general laws, which 

 may be premature, or may be rendered inad- 

 mis'sible by the existence of ascertained in- 

 dividual peculiarities, or more extensive dif- 

 ferences among various classes of animals. 



In the commencement of this treatise (p. 

 3.), the ovum of animals has been described 

 as consisting in general of two sets of parts, 

 of which the one is formed in the ovary of 



the female, and the other is superadded to 

 the first after it has left the place of its forma- 

 tion. The first, constituting the ovarian ovum 

 (or ovulum of some authors), consists essen- 

 tially of, 1, the external vesicular covering, or 

 vitelline membrane, 2, the yolk or vitelline 

 substance, and 3, the germinal vesicle, with, 4, 

 its nucleus or macula. As being the most con- 

 stant in their structure and relations, and the 

 more immediate seat and agents of the fofma- 

 tion of the embryo, these parts may be re- 

 garded as the essential and most important 

 structures of the egg. The other set of parts, 

 which are subject to great variation in dif- 

 ferent animals, and are only remotely con- 

 nected with the formative process, consist 

 chiefly of external coverings of more or less 

 density, either membranous or calcareous, 

 within which often is enclosed some portion 

 of albumen or albuminous fluid along with 

 the ovarian ovum. They serve the purpose, 

 in oviparous animals, of protecting the ovarian 

 egg from the hurtful influences of external 

 agents, and, in rarer instances, of supplying ad- 

 ditional nourishment to the embryo ; in vivi- 

 parous animals these parts (as in the chorion 

 of mammalia) serve more important ends, 

 being the more immediate means of establish- 

 ing that intimate organic connection between 

 the parent and embryo, by which a continual 

 supply of materials for the growth of the 

 latter, is transmitted into its body. From 

 analogy, the outermost covering of the ovum 

 of all animals has generally been termed the 

 chorion. fe 



In thus stating the general structure of the 

 ovum of animals, a certain amount of uni- 

 formity, analogy, or correspondence in their 

 several parts has in the meantime been as- 

 sumed ; and the best ascertained facts seemed 

 for long, indeed, to have warranted such an 

 assumption. It will be^for after consideration 

 to determine how far the comparison of the 

 several parts of the ova of different animals 

 proves their uniformity of structure and rela- 

 tion. In some instances it may appear difficult 

 or impossible to discover the correspondence; 

 and we must therefore be careful not to be 

 led by the attractive nature of a great genera- 

 lization, to confound a merely functional or 

 physiological analogy, with an anatomical, a 

 structural and relational identity. 



Without entering at present into a detail of 

 the observations bearing upon the foregoing 

 view of the general analogy in structure and 

 function of the ova of animals (which in 

 truth would involve the history of almost all 

 the more recent researches on the subject), 

 it seems proper at this place to call the at- 

 tention of the reader to the most prominent 

 modern discoveries which have laid the found- 

 ation of a more accurate acquaintance with 

 the structure of the ovum, and have also led 

 to the application of improved histological 

 views to the study of ovology.* 



* It is not intended at this place to refer to the 

 history of discovery in the department of our subject 

 which includes embryological development, in which 



