OVUM. 



[115] 



wards appears as a small dot or nucleolus. The 

 yolk begins, in the same manner as we have 

 had occasion to state in many other animals, 

 first by the clear deposit of a basement sub- 

 stance round the germinal vesicle, and the 

 subsequent formation of opaque granules in 

 it ; the vitelline membrane is of later for- 

 mation. As the egg increases in size, the 

 larger corpuscles and the fat globules gra- 

 dually appear. The ovarian ova of several 

 spiders contain besides the usual parts another 

 body of a peculiar kind, the nature of which 

 seems still involved in some doubt. This body 

 is eccentrically placed near the yolk mass of 

 the primitive ovum, and is of considerable 

 size, viz. about ?h/', of a yellowish colour, 

 and, during the earlier part of its existence at 

 least, consisting of concentric layers of a hard 

 granular matter. V. Cams* has compared this 

 body to the yolk-nucleus of the Frog's ovum ; 

 and both he and Von Siebold seem disposed 

 to consider it as in some way or other the 

 source of the granular substance of the yolk ; 

 but according to Wittich this view is not well 

 founded, as he has observed the body remain- 

 ing in the ovum till it reaches maturity, though 

 it loses its concentric laminated structure, and 

 becomes clearer and vesicular. Von Siebold, 

 on the contrary, states that it gradually disap- 

 pears. The large clear or oily globules appear, 

 according to Carus, to be produced from near 

 the pedicle of the ovum, at a place where there 

 is fixed a group of cells apparently destined 

 for their formation. 



No observations have as yet been made, so 

 far as I am aware, on the existence of a mi- 

 cropyle in the ova of Arachnida. 



Almost all the Arachnida are oviparous. 

 The Scorpions are an exception, however, 

 bearing their young alive ; and it is deserving 

 of notice that in this family the embryo is deve- 

 loped in the ovum while it still remains in the 

 ovary. In the greater number of this class em- 

 bryonic development commences in a blasto- 

 derm, which covers only a part of the surface 

 of the yolk, situated in what may be called its 

 lower part or pole } ; and the segmentation of 

 the yolk is therefore limited or partial, as in 

 Insects. In the higher Arachnida the steps of 

 this process do not appear to have been yet 

 satisfactorily observed. I may refer, however, 

 to the researches of Kaufuiann of Lucerne on 

 the development of the Tardigrada, as afford- 

 ing clear and beautiful illustrations of the 

 process of segmentation, which is shown to 

 be complete in the lower Arachnida.J 



Crustacea. All the animals of this class are 

 of distinct sex ; but in the allied Cirrhipedia 

 hermaphroditism most frequently prevails. In 

 some of the Cirrhipedes, however, it has been 

 shown by Mr. Darwin $ that the sexes are 



* Loc cit, p. 99. 



t See the Researches of Herhold De General. 

 Aranearum in Ovo, 1824 ; and Rathke, zur Mor- 

 phol. Reisebemerkung. 1837; and in Burdach's 

 Physiologie. 



I Zeitsch. fur Wissen. Zool., vol. iii., p. 220. See 

 Plate vi. f figs. 3. to 11. 



Monograph of the Sub-class Cirrhipedia, &c., 

 printed by the Ray Society, 1854, p. 27, &c. 



also distinct, as in some of the species of the 

 genera Ibla, Scalpellum, Alcippe, and Crypto- 

 phialus. In these instances the males are 

 very minute, and are attached, almost like pa- 

 rasites, to certain parts of the more developed 

 females, the place of their attachment varying 

 in different species. It is interesting to ob- 

 serve that these males, as in the case of several 

 of the Epizoa, are often of the most rudimen- 

 tary organisation.* 



The ova of the greater number of Crustacea, 

 especially the more highly organised genera, 

 belong, like those of most of the Articutata, to 

 the group in which a considerable amount of 

 nutritive yolk is present along with the for- 

 mative part, and in which the process of seg- 

 mentation in the latter is partial. The forma- 

 tive disc is situated on the lower surface of 

 the ovum ; and from that part the development 

 of the embryo emanates. Even among the 

 higher decapodous Crustacea, however, the 

 ova are of very various sizes -j-; and in the 

 lowest genera, as among the Entomostraca, 

 the ova are proportionally the largest, although 

 they are of the simplest structure, and present 

 the smallest amount of nutritive yolk ; so 

 that, in this as in other classes of animals, 

 magnitude alone is no true criterion of the 

 internal structure of the ovum. 



The ova of this class have been described 

 principally by Rathke J, by Erdl$, R. Wag- 

 ner |j , Leuckartl, Leydig**, and others ; but 

 the knowledge both of their structure and 

 their mode of formation is yet far from being 

 sufficiently minute or complete. They pre- 

 sent, indeed, many varieties, which renders it 

 difficult to give any general description of 

 them. The following may however be stated. 

 The ova of Crustacea are often variously and 

 brilliantly coloured. The yolk-substance con- 

 sists of a large quantity of clear globules of 

 considerable size, having the aspect of oil 

 globules, in which the colouring matter chiefly 

 resides. In some ova these globules attain 

 the size of ^^ of an inch. There is also a 

 more fluid granular matter in the yolk, and in 

 the more mature ova there is a layer or disc of 

 granular corpuscles on one side which after- 

 wards is the seat of segmentation and embry- 

 onic formation. The germinal vesicle is of 

 considerable size, in some instances possess- 



* It is also a remarkable fact, pointed out by Mr. 

 Darwin in his interesting Researches, that even 

 among the hermaphrodite species there are some- 

 times distinct male individuals attached parasiti- 

 cally to the hermaphrodite animals ; these have 

 been called complementary males. 



t Thus, for example, the ova of the river craw- 

 fish (Astacus fluviatilis) are twice as large as those 

 of the common lobster. 



J Entwickel. des Flusskrebses, 1829; in Bur- 

 dach's Physiologie, vol. ii. 1837 ; in his Abhandl. 

 zur Bildung und Entwickel. Gesch. &c., 1833 ; in 

 Dissert, de Animal. Crustac. Generat 1844 ; and 

 various other treatises. 



Entwickel. des Hummereies, 1843. 



|| Prodrom. Hist. Generat., 1836. 



*B" Article Zeugung. 



** On Argulus foUaceus in Zeitsch. fur Wissen. 

 Zool., vol. ii. 



