. The British woodlice, being a monograph of the terrestrial isopod Crustacea occurring in the British Islands. Crustacea -- Great Britain; Oniscidae; Anthropoda; Invertebrates. 10 THE BRI'lISH WOODLICE. parent, it is necessary for the young creatures to be well supplied with nutritive material. In fact, the bulk of the large Qgg is made up of food-yolk, on the outside of which the formative protoplasm is disposed in irregular patches. In the fertilized ovum, one of the latter, which lies in a particular position at the end, is found to be larger than the others (see fig. 22). It contains the
. The British woodlice, being a monograph of the terrestrial isopod Crustacea occurring in the British Islands. Crustacea -- Great Britain; Oniscidae; Anthropoda; Invertebrates. 10 THE BRI'lISH WOODLICE. parent, it is necessary for the young creatures to be well supplied with nutritive material. In fact, the bulk of the large Qgg is made up of food-yolk, on the outside of which the formative protoplasm is disposed in irregular patches. In the fertilized ovum, one of the latter, which lies in a particular position at the end, is found to be larger than the others (see fig. 22). It contains the nucleus of the egg-cell (see fig. 23) and is called the cicatricula. This is the only portion of the egg which divides and produces nucleated cells. It is these which gradually spread all over the surface of the food-yolk, forming a layer known as the blastoderm, which is at first but one cell thick (see figs. 24, 26, and 28). Before, however, the food-yolk is quite closed in, a differ- entiation into two layers—the pro-ectoderm and pvo-endoderm— takes place (see fig. 25) and rudiments of the first two pairs of ula. FIG. 22.—THE FERTILIZED EGG FIG. 23-—THE FERTILIZED EGG SEEN IN SECTION {PoictlUo scaber), after roule. (Porceilio scaber), after roule. appendages appear (see fig. 26). Moreover, the cells of the ectoderm change their shape and begin to multiply at two points to form the beginnings of the cerebral ganglia and the nerve cord respectively. As the blastoderm closes over the food-yolk, two more appendages arise and these are soon followed by others (see fig. 28). A depression appears at the point where the blastoderm closed and internally the pro-endoderm or inner layer is differ- entiated into two—the endoderm proper and the mesoderm (see fig. 29). The former begins to grow so that its edges unite to form the middle part of the intestine (see fig. 29) seen from the outside in fig. 30. The depression already mentioned grows deeper, forming a tube which is the
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectinverte, bookyear1906
