1 1 01] 



OVUM. 



dicle, he affirms consists only of the ovarian 

 structure, and of no part of the membranes of 

 the ovum. From his observations on Gaster- 

 osteus, in which the projecting bodies from 

 the porous or outer membrane in the vicinity 

 of the micropyle enable this part to be easily 

 recognised, he feels confident that if any 

 pediculated connection had existed it could 

 hardly have escaped notice. 



When the ovarian ovum has attained ma- 

 turity it falls into the cavity of the ovary, or 

 that which may be regarded as ovary and 

 oviduct united, by the rupture of the ovarian 

 capsule in which it is contained. The walls 

 of the ovi-capsules have by this time become 

 extremely thin ; but according to Von Baer a 

 small stigma or non-vascular mark may be dis- 

 tinguished where the rupture takes place. 

 After the ova have fallen into the common 

 cavity they are surrounded by a considerable 

 amount of secreted albuminous matter, by 

 which in some fishes the ova are covered 

 when excluded. In some this albuminous se- 

 cretion serves to unite the spawn in chains or 

 networks. In other fishes the ova are covered 

 externally with villous projections ; but the 

 manner in which these are formed has not 

 yet, so far as I am aware, been observed. 



One of the most remarkable, but as yet 

 quite unexplained, varieties in the external 

 coverings of the ovum in one of the osseous 

 fishes, is that discovered and recently de- 

 scribed by Ernst Hackel, as occurring in the 

 family of Scomberesoces.* This consists in 

 the formation, in the space between the sur- 

 face of the 3'olk and the vitelline membrane 

 (that is, the porous membrane), of a layer of 

 long and very distinct fibres, which are wound 

 somewhat spirally, but irregularly, over the 

 surface of the yolk. Hackel has traced the 

 gradual formation of these in fresh specimens 

 of Belone from points on the surface of the 

 yolk-substance; and in other genera he has 

 observed several varieties in the forms of the 

 fibres. They are on an average about a^Vo" 

 thick, and long enough to surround the egg 

 several times; and they appear to resemble the 

 fibres of the elastic yellow tissue more than 

 any other animal substance, but do not entirely 

 agree with them. In the meantime we must 

 suspend our judgment as to this very extraordi- 

 nary addition to the surface of the ovum until 

 farther observations shall have been made as 

 to their distribution in various fishes or other 

 animals, and as to their relation to the deve- 

 lopment of the embryo. -J- 



* MUller's Archiv. 1855, p. 23. See plates IV. 

 and V. 



t Some time after the above was in the hands of 

 the printer, I received the first and second parts of 

 the seventh volume of the Zeitsch. fur Wissen. 

 Zool., containing a notice of the discovery of the 

 micropyle in the Salmo salar, and S. fario, by 

 Professor Bruch of Basle. The observations leading 

 to this discovery were made in the winter of 1854-5; 

 and it is right to state here, that Dr. Ransom's dis- 

 covery of the micropyle in the gasterosteus, which 

 was communicated to the Royal Society on the 23rd 

 of November, 1854, was made in the months of June 

 and July previous; and these observations had been 



Invertebrate Animals. The ova of Inverte 

 brata may be considered under two princi- 

 pal divisions, according as they present more 

 of the large-celled or of the finely granular 

 yolk-substance. The ova of the first kind 

 are usually of a larger size ; they possess a 

 larger germinal vesicle, and often a divided 

 or multiple macula ; and the process of seg- 

 mentation in them is either partial, that is, 

 limited to one part of the surface of the yolk, 

 or it occurs in a different manner on the upper 

 and lower sides of the ovum. In these there 

 is, in fact, nutritive as well as formative yolk. 

 In the other division of animals the yolk is 

 finally molecular, or is mainly composed of 

 smaller granules, and is chiefly of the formative 

 kind ; segmentation usually involves the whole 

 yolk, or if not so, is very nearly complete : 

 the germinal vesicle is generally clear, and 

 the macula most frequently single, and well 

 marked. It is true that the form and struc- 

 ture of the ova of Invertebrata presents many 

 and considerable varieties, as might indeed be 

 expected among animals of such diversity of 

 organisation as belongs to the great divisions 

 of the Radiata, Articulata, and Mollusca ; 

 but still it is to be observed that as a greater 

 degree of simplicity exists in the form and 

 structure of the primordial elements than in 

 the more developed textures and organs of ani- 

 mals, so also we find that much closer analogies 

 may be traced among these elements in the 

 lowest classes of the animal kingdom. We 

 meet, therefore,with little difficulty, even in the 

 most diverse tribes of the Invertebrate animals 

 in tracing the correspondence of the essential 

 parts of the ovum; and we are enabled also to 

 trace a more close analogy between these and 

 the corresponding parts in the Vertebrata 

 than might have been expected. We are there- 

 fore warranted in applying to them similar 

 designations ; and we have daily increasing 

 reason to trust to observations made on the 

 ovology of the lower animals as the means of 

 extending the knowledge of the reproductive 

 functions in Vertebrata and in Man. Thus 

 the recent discovery of the micropyle aperture 

 in some animals, and the certain and clear ob- 

 servation of the penetration of the sperma- 



commnnicated to Professor Sharpey and myself in 

 August and September. In the beginning of 

 January, 1855, Dr. Ransom informed me by letter 

 of his having found the micropyle also in the Trout, 

 and a few days later in the Salmon. I then saw the 

 micropyle in the ova of both of these fishes; and I 

 have since examined it minutely in the Stickleback, 

 and have confirmed in every particular Dr. Ransom's 

 statements. The existence of the raicropyle in these 

 Vertebrate animals has thus been established by 

 several independent observations ; and I believe that 

 no one who uses the proper means can fail to detect 

 it in these and other fishes. Professor Bruch's ob- 

 servations were chiefly made on the ova after im- 

 pregnation, which may explain the reason of his 

 having failed to perceive the connection pointed out 

 between this aperture and the depression in the 

 centre of the germ disc. Bruch was like myself 

 unsuccessful in perceiving the entrance of sperma- 

 tozoa by the micropyle. His measurement of the 

 micropyle in the Salmon and Trout does not agree 

 with mine, making it much smaller. 



