[114] 



OVUM. 



Caryse, and of his views as to the nature of 

 this process of non-sexual reproduction in 

 general. The viviparous Aphides, according 

 to Dr. Burnett, are neither male nor female, 

 and do not possess, as has been supposed, 

 any ovaries or oviducts. The new colony 

 already begins to be visible within the body 

 of its parent before the latter has itself been 

 brought forth. The substance in which the 

 new progeny takes its origin consists, at first, 

 either of a single nucleated cell of ^Vo" m 

 diameter, or of a small mass of these cells at- 

 tached in the same place as that occupied by 

 the ovary in the oviparous females. These 

 masses increase in quantity, are subdivided by 

 a kind of notching into more numerous 

 masses ; and each of these being inclosed in a 

 capsule, the whole come to be arranged in a 

 continuous row or series. There is not, 

 however, any germinal vesicle nor segmenta- 

 tion, as in the sexual ova ; and when develop- 

 ment of the new insects is complete, it is by 

 falling into the abdominal cavity, and by es- 

 caping through a genital aperture (poms 

 genitalis) that the offspring is excluded. 



With regard to the origin of the cellular 

 mass or germ from which the non-sexual 

 progeny proceeds, Dr. Burnett states that a 

 small mass, of a different appearance from the 

 germinal part of the ovum, is seen to be in- 

 cluded within the arches of the embryo ; and 

 the next colony is produced from this mass. 

 He regards this process as analogous rather 

 to one of internal gemmation than of true 

 generation, coinciding therefore more nearly 

 with the views of Leuckart and Carpenter 

 than of Steenstrup and Owen. 



Arachnida. The ova of nearly all the 

 higher Arachnida do not differ much in their 

 internal structure from those of Insects ; but 

 they do not present the same varieties of ex- 

 ternal form. Their mode of first origin is 

 also very different. All the higher Arachnida 

 are, like Insects, of separate sexes. The Tar- 

 digrada are hermaphrodite; and in these as 

 well as some other simpler Arachnida, as 

 Pycnogonida and Acari, the ovum, though 

 proportionally of large size, is of extremely 

 simple structure, approaching very nearly to 

 that of the lowest classes of Invertebrate 

 animals. 



The ova of the higher Arachnida are gene- 

 rally spheroidal ; the chorion or external 

 membrane is generally smooth ; the vitelline 

 membrane is slender, clear, and structure- 

 less ; the yolk-substance is not unfrequently 

 coloured, often purplish, consisting of a consi- 

 derable quantity of large oily-looking globules, 

 smaller granules of various sizes, and larger 

 corpuscles which have been looked upon as 

 cells, but which Leuckart states are only ag- 

 gregated masses of granules held together by 

 a viscid substance. The germinal vesicle is 

 proportionally large, placed eccentrically, and 

 possesses a macula, which in some genera is 

 simple and flattened, as in the Scorpion, in 

 others multiple and granular, as in Epeira. 



The formation of the ova may be observed 

 in Arachnida with great ease, from the manner 



in which they are disposed in the ovary, pro- 

 jecting like bunches of grapes from the central 

 part of that organ, in almost every stage or 

 degree of advancement. The process has 

 been carefully observed by Wittich * and 

 others. So soon almost as the ovum begins 

 to be formed, it causes a bulging or projec- 

 tion of the membrane from the surface of the 

 ovary ; and when that has somewhat increased 

 in size, the ova hang or project from the sur- 

 face in small pcdiculated ovi-capsules. Ac- 

 cording to Wittich, V. Carus f, and Leuckart, 

 the part of the ovum which earliest makes its 

 appearance within the small ovicapsules is the 

 germinal vesicle. At first it appears quite sim- 

 ple and without a macula, which last soon after- 



Fig. SO*. 



Ovarian ova of the Spider. ( From Wittich.') 



a. Small fragment of the ovary of Epeira diadema 

 from which three ova project in the early stage of 

 their development previous to the formation of 

 the yolk : the germinal vesicles are enclosed in the 

 membrane formed by the bulging out of the ovarian 

 substance. 



b. Two ova similar!}' situated, but more advanced ; 

 the primitive granular yolk substance intervening 

 between the germinal vesicle and vitelline mem- 

 brane. 



c. An ovum still more developed ; the germinal 

 vesicle occupies the upper part; in the finely gran- 

 ular yolk substance below is seen the dark body 

 regarded by some as a yolk nucleus, presenting an 

 appearance of concentric lamellar structure ; towards 

 c. in the figure, or close to the connecting pedicle, 

 the large nucleated cells are seen, which usually 

 occupy that situation, and appear to give rise to the 

 cellular yolk substance. 



d. More advanced ovum greatly increased in 

 bulk, the pedicle diminished, and the yolk com- 

 pletely occupied by the larger cells or corpuscles ; 

 the yolk nucleus has disappeared or is obscured. 



e. f. Different forms of the yolk nucleus or 

 dark body, which for a variable time is placed 

 within the ovum during its formation. 



* Die Entstehung des Arachnideneies im Eier- 

 stock, &c., Miiller's Archiv. for 1849, p. 113. 

 f Zcitsch. fur Wissen. Zool., vol. ii., 1850, p. 97. 



