Eighth period.—The exterior parts which are developed in the cambium, viz. the feet, the mandibles, and the head, are more neatly de fined. The ovum (fig. 110) now presents a very important pecu liarity, but which was in some measure in dicated in the pre ceding period. Its size is slightly di minished anteriorly, and the vitellus con sequently is divided into two portions.
The smaller and an terior part (a) is rea dily distinguishable from the dorsal part of the fcetus, and occupies the place which sub sequently becomes that of the corslet ; M. Heroldt consequently terms it the thoracic re gion. The other part is the abdominal region, which is very conspicuous, occupies more than one-half. the bulk of the ovum, and seems to constitute the greatest portion of the abdomen. If the inferior surface of the abdominal region be examined, there will be seen, in addition to a spot which ornaments that part, some addi tional oblique and curved folds, which indicate the formation of the integuments; another and a more important change has now taken place on the middle line of the superior surface; viz. an obscure straight band (b) which commences at the thoracic-abdominal constriction, and reaches to the extremity of the ovum, becoming gradually narrower in that direction. This band, which does not give off lateral processes in any part of its course, is to be considered as the rudiment of the heart or dorsal vessel. The fluid which it doubtless contains in its interior is motionless. Heroldt thinks that the forma tion of the fluid is anterior to that of the parietes in which it is enclosed : he also be lieves that it is the albumen which gives origin to the circulatory apparatus, and further attri butes to it the origin of all the integuments. These are, doubtless, important questions to solve, but as they are the result of speculation rather than direct observation, we have deemed it proper to omit the theories by which they are supported, and confine ourselves to a simple enunciation of the facts The eyes (d) are now distinguishable.
Ninth period.—.The ovum presents a more sensible diminution anteriorly, and is more dis tinctly divisible into two parts. The anterior and narrow portion constitutes the smaller ex tremity, and includes the head, the thorax, and their appendages ; the other portion, which is spherical and of much larger size, constitutes the greater extremity and corresponds to the abdomen. At the same time that these modi fications take place the ovum becomes slightly elongated, and all the parts which can be dis tinguished therein have proceeded towards their perfection. The legs now present slight
traces of a division into joints, and they have increased so far in length that they cover almost the whole of the lower surface of the thorax.
Tenth period.—The small extremity, which is still more elongated, is now found to be dis tinguished from the large one by a true con striction, dividing the ovum into the parts de nominated in the perfect spider ' thorax' and abdomen.' The visible parts of the thorax are the mandibles, the palpi, and the legs ; these latter appendages are folded upon the chest, and have grown so long as to cross the middle line of the body; they are locked in the interspaces of each other, like the fingers when the hands are clasped together. The abdomen presents nothing remarkable, except the elon gated opake streak which exists along the middle of the inferior surface from the feet to the termi nation of the abdomen, and which was already visible at the preceding periods. ( Fig. 110, e.) Heroldt imagines this streak to be an indication of the development of the internal parts of the abdomen, viz. the intestinal canal, the secreting vessels of the web, and the genital organs, &c. In proportion as the fcetus increases, the ex ternal membrane or covering of the egg is ap plied more exactly to its body, and seems to represent an exterior skin, of which the young spider soon divests itself, almost in the same manner as the caterpillar sheds the skin in which it is enveloped.
Eleventh period.—By the progressive in crease of the fcetus the membrane of the egg becomes so much stretched, and is applied so exactly to the surface of the body of the animal that the different parts can be distinctly seen through it, like the nymph or chrysalis of certain coleopterous insects. The essential parts of the thorax are the head and the feet. The head is of a white colour, and is surmounted by eight brown streaks ; the legs, which are also white, are closely applied to the chest, with their extre mities alternating with each other. One may dis tinguish in each a hip, a thigh, a leg, and a tarsus. The articulations of the palps and mandibles are also visible through the general envelope of the egg. The inferior streak of the abdomen is much more extended, and seems to be divi ded into two parts, one large and elliptical in figure, the other small and rounded; the latter corresponds to the anal aperture ; at this last stage of the development, the fcetus or the im prisoned young spider, as it may be called, gives no sign of motion.