Memoirs Read Before the Boston Society of Natural History Volume 3

Memoirs Read Before the Boston Society of Natural History Volume 3

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This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1894 edition. Excerpt: ... of the median invagination there are to be seen, in the region of the future side cords, two lateral grooves. The cells in these regions have the characters of the cells invaginated for the middle cord and they thus indicate the origin of the two lateral cords as invaginations of the superficial ectoderm, and not as linear tracts of cells budded off from the inner ends of the epithelial cells of the germinal band, as Hatschek has figured and described for Lepidoptera (loc. cit., pl. 1, fig. 6, p. 8). The sections of the stages in which these cells assume their characteristic appearances show them occupying the region of the future lateral cords, but not confined within such-definitely marked areas as Hatschek figures. Before the appearance of the appendages the mesoderm is seen to lie as a continuous, thick layer of cells apposed to the germinal band. This condition persists in the abdominal region after the thoracic appendages have grown out, but disappears before the invagination for the nervous system has extended itself into this region. (P1. 22, figs. 27 and 28.) It divides into two lateral plates as the invagination advances. In their outer halves these lateral plates of mesoderm are split into two sheets which contain between them the primitive body cavity (pl. 23, fig. 13, l'and figs. 11 and 12, bc), but the latter soon disappears by the fusion of its walls. The separation of the splanchnic and somatic layers of the mesoderm occurs before the invagination for the middle cord, but after the formation of the lateral cords of the nervous system. The primitive body cavity exists in the form of a pair of tubes extending from the head region backward for a greater or less distance; but on account of the retardation in the development of.show more

Product details

  • Paperback | 274 pages
  • 189 x 246 x 15mm | 494g
  • Rarebooksclub.com
  • United States
  • English
  • black & white illustrations
  • 1236777565
  • 9781236777560