• Anglický jazyk

The Evolution of Man Volume 1

Autor: Ernst Heinrich Philipp Haeckel

Excerpt: ...capable of being reduced to one common type of embryonic development; and this primitive type is most certainly exhibited to-day by the amphioxus. It must, therefore, be our next task to make a comparative study of the various forms of vertebrate... Viac o knihe

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Excerpt: ...capable of being reduced to one common type of embryonic development; and this primitive type is most certainly exhibited to-day by the amphioxus. It must, therefore, be our next task to make a comparative study of the various forms of vertebrate gastrulation, and trace them backwards to that of the lancelet. Broadly speaking, they fall first into two groups: the older cyclostoma, the earliest fishes, most of the amphibia, and the viviparous mammals, have holoblastic ova-that is to say, ova with total, unequal segmentation; while the younger cyclostoma, most of the fishes, the cephalopods, reptiles, birds, and monotremes, have meroblastic ova, or ova with partial discoid segmentation. A closer study of them shows, however, that these two groups do not present a natural unity, and that the historical relations between their several divisions are very complicated. In order to understand them properly, we must first consider the various modifications of gastrulation in these classes. We may begin with that of the amphibia. The most suitable and most available objects of study in this class are the eggs of our indigenous amphibia, the tailless frogs and toads, and the tailed salamander. In spring they are to be found in clusters in every pond, and careful examination of the ova with a lens is sufficient to show at least the external features of the segmentation. In order to understand the whole process rightly and follow the formation of the germinal layers and the gastrula, the ova of the frog and salamander must be carefully hardened; then the thinnest possible sections must be made of the hardened ova with the microtome, and the tinted sections must be very closely compared under a powerful microscope. The ova of the frog or toad are globular in shape, about the twelfth of an inch in diameter, and are clustered in jelly-like masses, which are lumped together in the case of the frog, but form long strings in the case of the toad. When we examine the...

  • Vydavateľstvo: Books LLC, Reference Series
  • Rok vydania: 2013
  • Formát: Paperback
  • Rozmer: 246 x 189 mm
  • Jazyk: Anglický jazyk
  • ISBN: 9781153701952

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