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, somewhatsimilar 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 ovum. All the parts which it contains seem subordinate to it, as we shall see by carefully tracing their development. A remarkable fact observed by Heroldt 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 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.
First period.—The im pregnated ovum being de posited, and the temperature being favourable, develop ment commences. The changes always begin at the margins of the cica tricula, which appear to be resolved into granul es,wh ich 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 (fig. 106, a), which has continued to change its 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 cornet 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 cornet 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 ance, through which, however, the globules of the vitellus may be distinguished. A single point remains perfectly transparent, and this is observed at the extremity of the egg (fig. 107, a,) opposite to that which the germ occupied after its displacement. Ileroldtcalls this clouded albumen colliquamentum Up to this period the vitellus seems not to un dergo any change ; all that has been hitherto observed takes place in the albumen and in the circular space between the yolk and the shell.
Sixth period. — The colliquamentum, or clouded albumen, which was extended over the yolk so as to conceal it, is now trated upon the point last occupied by nucleus of the germ, and has assumed a pearly colour (fig. 108). Its consistence is pretty
solid ; it is opake, so that the globules of the yolk I can no longer be guished through it, b though they are elsewhere more conspicuous on count of the retreat of the clouded albumen towards this single point; from this moment the mentum, which seems to have changed its nature, receives a new name, and is designated by Heroldt the cambium. The cambium covers more than a fourth part of the circumference of the yolk; its form is already pretty well marked, and two parts may be distinguished in it ; one large (b), the other small (a), which are separated by a kind of contraction. The form of the larger division is elliptical, and it is in its substance that the thorax, the legs, and the essential internal parts of the foetus will soon be perceived to develope themselves. The smaller division is of a rounded form, and seems, as it were, an appendage to the preceding; it is destined to give origin to the head, the gans of sense, and the appendages of those of mastication. So much being premised, we may call, with Heroldt, the larger division cambium thoracicuin, the lesser one cambiunt cephalicum. We may also, for the better comprehension of the changes which are about to succeed each other, divide the superficies of the ovum into four regions. That which contains the cambium may be called the pectoral region, the opposite portion may be called the dorsal, and the two intermediate parts the lateral regions. We may observe that in other species of aranew where the ova are spherical, the germ is at once converted into mentum, and then into cambium, without a change of situation. The Aranca diadema offers an example of this circumstance ; in other spects there is no important difference observable. Seventh period.—The two portions of the cambium, the cephalic and thoracic, have as yet presented only the appearance of an opake and homogeneous mass, but now we may distinguish traces of rings, four in number on either side ; these are the rudiments of the legs. ( Fig. 109, 1, 2, 3, 4.) They occupy the lateral aspects of the anterior part of the ovum ; they are also visible on the pectoral region, towards which they are prolonged in feriorly. The extre mity of the first leg is contiguous to that of the opposite side ; but the three others, though of greater length, yet do not reach so low down, but leave a triangular interspace between them, which is filled with a cloudy and somewhat transparent matter, through which the vitelline globules are visible. This triangular space, which is subsequently to be covered by the legs, seems to give origin to the trunk and to many parts contained in the abdomen. In tracing the two portions of the cambium through the changes which they have undergone, we find that the thoracic portion is represented by the legs and their intermediate space, and that the cephalic por tion is anterior to this. The alterations of the latter part are not less remarkable ; instead' of being rounded anteriorly it is truncated, and we may perceive a ring at the sides, which is not divided on the inferior middle line of the body, and which represents the maxillary palps (b). One may even distinguish, as if through a cloud, the rudiments of the mandibles. It is probable that all the parts which to the head, as the eyes, the mandibular hooks, and the maxillae, have their limits well defined from this period. With respect to the head, (a) it is neatly separated from the chest ; and this fact it is of importance to dwell on, since in all the full-grown spiders the conflu ence of the two parts is most intimate, and their original separation only indicated by a groove of greater or less depth. The ovum, also, now presents some other new appearances ; these are a kind of furrows or arched folds (c c), which are seen on the vitellus behind the legs ; and which deserve attention, since they indicate the formation of the common teguments of the foetus. And we must here observe that the parts which are already de veloped have an intimate connection with the vitellus. Thus if an ovum be opened with all the precautions requisite for so delicate an operation, and if the matter of it be extended on a piece of glass, we see that the parts formed in the cambium preserve their general figure, and that the internal layer of the mucous and whitish matter of which it consists is in intimate communication with the vitellus. It is implanted upon the yolk just as fungi and other parasitic plants are attached to the trunk of a tree : the yolk, then, is subservient to the nutrition of the most exterior parts of the body.