212 



ARACHNIDA. 



104. 



representing a vertical 

 section of a fecundated 

 ovum at the moment 

 of exclusion, and before 

 any organ has been de- 

 c veloped. The vitellus 

 or yolk () forms the 

 greatest part of the 

 contained liquid mat- 

 ter, and the egg is 

 almost entirely filled by it : its colour is gene- 

 rally that of yellow ochre, and sometimes has 

 a saffron tinge. In some species the yolk is 

 grey, white, or reddish brown; and in each 

 case the colour of this part determines the ge- 

 neral tint of the egg. If the yolk be consi- 

 derably magnified, it is seen to be composed of 

 an infinite number of minute globules of various 

 sizes, swimming in the albumen, or surrounded 

 by it, and resembling so many small yolks. 



The albumen (b) is a transparent crystalline 

 liquid, without distinct organical parts, and 

 consequently not presenting any globules, sur- 

 rounding the vitellus as far as the cicatricula, 

 and intermediate in bulk or quantity to these. 

 If an ovum be opened, and the liquid which 

 it contains be poured out upon glass, the albu- 

 men is seen to surround the globules of the 

 vitellus and cicatricula exactly as the serum 

 of the blood envelopes the crassamentum. 

 In the interior of the egg the albumen is 

 situated, like the cicatricula, externally to the 

 yolk, and fills the interspace between the yolk 

 and the exterior membrane of the egg. It is 

 in this interspace that the first lineaments of 

 the embryo appear, and here the head, thorax, 

 members, integuments, and their appendages 7 , 

 and all the internal organs, without excepting 

 the intestines, are successively developed. 



The cicatricula or germ (c) is the smallest 

 and most important part of the ovum. It is 

 situated immediately beneath the exterior co- 

 vering, and at the centre of the circumference 

 of the egg. It is distinguished by the naked 

 eye in the form of a little white point. If it 

 be examined with more care, we perceive that 

 it is of a lenticular figure, and is composed 

 of an innumerable quantity of whitish granules. 

 Under the microscope these granules are seen 

 to be of a globular figure, somewhat similar in 

 this respect to those of the yolk, but more opake, 

 and of a smaller diameter. When segregated 

 and diffused they present a striking analogy 

 to the grains of pollen, but with this difference, 

 that the pollen of vegetables is composed of 

 vesicles filled with organic molecules, whilst 

 each of these globules of the cicatricula must 

 be regarded as simple. The cicatricula or 

 germ is the centre of radiation of all the 

 changes which take place in the ovnm. All 

 the parts which it contains seem subordinate 

 to it, as we shall see by carefully tracing their 

 development. A remarkable fact observed by 

 Ileroldt in the ova of some undetermined 

 species of spiders is this, that in place of a 

 single cicatricula, there appear to be several 

 spread over different points of the surface of 

 the ovum ; but these small germs rapidly 

 coalesce into one mass, which soon assumes 



the ordinary form of the single cicatricula. 

 The component parts of the ovum being known, 

 we proceed to the metamorphoses which they 

 undergo up to the time when the young spider 

 breaks through the shell. 



Fig 105 First period. The im- 



^Bljg^sjJ pregnated ovum being de- 



posited, and the temperature 

 being favourable, develop- 



The 



'o 



ment 



commences. 



begin 



at 



changes always 

 the margins of the cica- 

 tricula, which appear to be 

 resolved into granules,which 

 extend into the albumen and 

 vitellus. The centre of the germ remains the 

 same, the only appreciable difference is the 

 enlargement of its circumference: (A, gives 

 the natural size of the ovum.) 



Second period. The germ is much larger, 

 its margins are dispersed in numerous granules ; 

 the centre is not yet affected by this tendency 

 to molecular dispersion, but has undergone 

 a notable modification. It changes its situation 

 and begins to move towards the extremity of 

 the ovum, leaving in the place which it for- 

 merly occupied a train of globules; it now, 

 to compare small things with great, bears some 

 resemblance to a comet, the nucleus of which is 

 represented by the centre of the germ ; the tail, 

 which is formed by the dispersion of the globules, 

 is transparent, and the vitellus which it covers 

 may be as distinctly seen through it as the fixed 

 stars through the tail of a comet. 



Third period. The nucleus of the germ 

 (Jig. 106, a), which has continued to change its 

 Fig. 106. place, is arrived near the 



extremity of the ovum, but 

 has not quite reached it. 

 The tract which it has 

 traversed is marked by an 

 infinity of granules, which 

 are then so much dissemi- 

 nated that they extend al- 

 most to the opposite extre- 

 mity of the ovum. It is 

 then that the kind of comet which it represents 

 is seen at its greatest development, and with all 

 the characters that have been indicated. The 

 movement of the nucleus of the cicatricula 

 authorizes the supposition that that body has 

 not, at least at the earlier periods, a very 

 intimate connexion with the vitellus. 



Fourth period. The nucleus of the germ 

 has not gone beyond the point which it had 

 attained, but it presents a new change. The 

 molecules are disseminated into an infinity of 

 granules, nothing remains of the comet but 

 the tail, which is still more extended ; but 

 we see then that the granules dispersed in 

 the albumen have a tendency to reassemble at 

 the point where the germ was originally situated. 

 Fifth, period. The germ of the ovum, which 

 appears to be disseminated in the albumen, 

 has undergone a very curious transformation. 

 Its nucleus has disappeared, all its granules 

 are decomposed into almost imperceptible mo- 

 lecules, which, in destroying the limpidity of 

 the albumen, have given it a clouded appear- 



