OVUM. 



[ 484 ] 



OXYRRHIS. 



man and mammalia averages about 1-150 

 to 1-200" in diameter, being rarely 1-100". 

 Another peculiarity consists in their ova, in- 

 stead of being in immediate contact by means 

 of their chorion or outer envelope, with the 

 stroma of the ovary, or being loose within 

 the cavity of the latter, as in other animals, 

 are enclosed in distinct larger cells, — the 

 Graafian vesicles. 



When the ovum of the mammalia leaves 

 the ovary, a portion of the proligerous disk 

 is seen to be adherent to it. In other ani- 

 mals new layers are secreted upon the out- 

 side of the ova by the oviduct or ovary, as 

 in the eggs of birds, insects, &c. 



On the escape of the ovum from the ovary, 

 the phsenomena which ensue vary according 

 to whether the ovum has been impregnated 

 or not. In both cases the germinal vesicle 

 and spot disappear; an interspace, filled 

 with albuminous liquid, occurs between the 

 yolk and the zona pellucida ; the ovum be- 

 comes covered with cilia, and undergoes a 

 regular motion of rotation, and certain move- 

 ments and changes in form of the yolk-sub- 

 stance, which forms Amceba-like processes, 

 have been noticed. In the unimpregnated 

 ovum, decay and decomposition subsequently 

 take place. 



The essential part of the process of im- 

 pregnation is the perforation of the vitelline 

 membrane by the spermatozoa, and their 

 entrance into the yolk, in which they subse- 

 quently dissolve. 



In the impregnated ovum, the germinal 

 vesicle soon disappears, the chorion becomes 

 thinner, the ovum grows, and the yolk begins 

 to undergo the process of segmentation ; 

 but just before this process commences, one 

 or two globules separate from the substance 

 of the yolk, being apparently pressed out of 

 it, and occupy the interspace between the 

 yolk and the chorion; these globules subse- 

 quently dissolve in the liquid. 



The process of segmentation has been de- 

 scribed under Cells (p. 115); but according 

 to another account, it takes place thus : — at 

 first a notch or slight indentation appears on 

 some part of the surface of the yolk ; this 

 becomes deeper and deeper, so as to encircle 

 the yolk with an annular depression. Soon 

 after the commencement of this, a clear spot 

 appears in the centre of each circumscribed 

 portion of the yolk. The depression be- 

 coming deeper, the yolk is divided into two 

 distinct portions. The process is continued 

 in the case of each of these in exactly the 

 same manner, and in that of the segments 



arising from their subdivision also, each 

 simultaneously acquiring a clear spot, until 

 the yolk appears entirely composed of innu- 

 merable small bodies, having the appearance 

 of nucleated cells. Finally, these become 

 very minute, and the yolk acqviires much 

 the appearance it had before impregnation. 

 Cells then form in the yolk, as in an ordi- 

 nary blastema, from without inwards, and 

 from the spot originally occupied by the 

 germinal vesicle as a centre, and from these 

 the tissues of the embryo are formed. 



According to this description, which is 

 most probably correct, the segmentation is 

 not a process of cell-division or endogenous 

 cell-formation, and the nuclear spots would 

 correspond to portions of the yolk- sub stance 

 from which the granules and globules of 

 sarcode were absent. 



In unimpregnated ova, segmentation takes 

 place to a certain extent, but irregularly and 

 incompletely. 



In the impregnated ova of some animals, 

 as in some of the Batrachia, most fishes and 

 Cephalopods, the segmentation is only par- 

 tial, a portion of the yolk remaining as at first. 

 The study of ova and their changes is very 

 difficult. The most favourable objects for 

 the purpose exist perhaps in those of the 

 aquatic Mollusca ; the ova of insects, as the 

 large species of Musca, of species of Pulex, 

 &c,, are also easily accessible. Some import- 

 ant results have been obtained with the ova 

 of the frog (frog's spawn). 



BiBL. KoWiker, Mikr.Anat.ii.; Al. Thom- 

 son, Cycl. Anat. Sfc, art. Ovum ; Vogt, Phy- 

 siol. Brief e ; Keber, De sperm, intr. in ovtila; 

 Bischoff", Widerlegung des v. Keber behaupt. 

 Eindringens d. Sperm, in das Ei; id. Be- 

 st'dtigung d. von Newport behaupt. Eindring. 

 Sfc, and numerous other memoirs; New- 

 port, Phil. Trans. 1851 and 1853 ; Siebold, 

 Vergleich.Anat.; Wagner, Elements of Phy- 

 siology, by Willis ; V. Beneden, Ann. d. Sc. 

 Nat. 3 ser. xiii. 



OXALATES. See the bases. 

 OXYGONIUM, Presl.— A genus of Di- 

 plasiepe (Polypodfeous Ferns). Exotic. 



OXYRRHIS, Duj.— A genus of Infusoria, 

 belonging to the family of Thecamonadina. 

 Char. Body ovoid-oblong, rugose, ob- 

 liquely notched in front and prolonged into 

 a point ; several flagelliform filaments arising 

 laterally from the bottom of the notch. 



O. marina (PL 24. fig. 54). Body colour- 

 less, subcvlindrical, rounded behind; marine; 

 length 1-500". 



BiBL. Dujardin, Infus. p. 34/. 



