. The Cambridge natural history. Zoology. ISO EMBRYOLOGY of division or arrangement of the body into segments appears very early, as shown in our Figure of some of the early stages of development oi Una (a beetle), Fig. 81. In A the segmen- tation of the ectoderm has not commenced, but the procephalic lobes {P (J) are seen; in B the three head segments are distinct, while in C the thoracic segmentation has occurred, and that of the abdomen has commenced. Graber considers that in this species the abdomen consists of ten segmental lobes, and a terminal piece or telson. According to G-raber ^ thi
. The Cambridge natural history. Zoology. ISO EMBRYOLOGY of division or arrangement of the body into segments appears very early, as shown in our Figure of some of the early stages of development oi Una (a beetle), Fig. 81. In A the segmen- tation of the ectoderm has not commenced, but the procephalic lobes {P (J) are seen; in B the three head segments are distinct, while in C the thoracic segmentation has occurred, and that of the abdomen has commenced. Graber considers that in this species the abdomen consists of ten segmental lobes, and a terminal piece or telson. According to G-raber ^ this is not a primitive condition, but is preceded by a division into three or. Fig. 81.—Early stages of the segmentation of a beetle {Lino): A, segmentation not visible, 1 day ; B, segmentation of bead visible ; C, segmentation still more advanced, 2J days ; PC, procephalic lobes ; g^, g-, g^, segments bearing appendages of the head ; ih, thorax ; tli}, th?, tli-', segments of the thorax ; a}, C(?, anterior abdominal. four parts, corresponding with the divisions that will afterwards be head, thorax, and abdomen. This primary segmentation, he says, takes place in the Hypoblast (Endoderm) layer of the ventral plate; this layer being, in an early stage of the development of a common grasshopper {Stenohothrus variaMlis), divided into four sections, two of which go to form the head, while the others become thorax and abdomen respectively. In Zina the primary segmentation is, Graber says, into three instead of four parts. Graber's opinion on the primary segmentation does not appear to be generally accepted, and Wheeler, who has studied' the 1 Morph. Jalirb. xiv. 1888, p. 345. - J. 3Iorphol. viii. 1893, p. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harmer, S. F. (Sidney Frederic), Sir, 1862- ed; Shipley, A
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
