United States or Suriname ? Vote for the TOP Country of the Week !
At an early stage in the development of the embryo, the cells composing it become divisible into three layers. It is even possible, as Loeb maintains, that this differentiation is present in the unsegmented ovum, in which case the facts to be detailed become still more remarkable and significant.
Gegenbaur referred on many other occasions to the close blood-relationship of the Tunicates and Vertebrates, and luminously explained the reasons that justify us in framing the hypothesis of the descent of the two stems from a common ancestor, an unsegmented worm-like animal with an axial chorda between the dorsal nerve-tube and the ventral gut-tube.
Thus the chief advantage in organisation by which the earliest Vertebrates took precedence of the unsegmented Chordonia consisted in the development of internal segmentation. The whole vertebrate stem divides first into the two chief sections of Acrania and Craniota. The Craniota descend directly from the Acrania, and these from the primitive Chordonia.
Man's ancestors are round coenobia or colonies of Protozoa; they consist of a close association of many homogeneous cells, and thus are individuals of the second order. Their body consists merely of a primitive gut, the wall of which is made up of the two primary germinal layers. The unsegmented chorda develops between the dorsal medullary tube and the ventral gut-tube. Head-gut with gill-clefts.
Undoubtedly both the Tunicates and Acrania have inherited the chorda from a common unsegmented stem-form; and these ancient, long-extinct ancestors of all the chordonia are our hypothetical Prochordonia.
These quit the egg with an unsegmented ovate body, a median frontal eye, and three pairs of natatory feet, of which the anterior are simple, and the other two biramose in fact, in the larval form, so common among the lower Crustacea, to which O.F. Muller gave the name of Nauplius.
Still, among these animal organs, the lower systems will lead in point of time. The brain must to a certain extent wait for the skeleton. The skeleton. The axial skeleton consists, in the lowest fish, of the notochord, a cylindrical unsegmented rod of cartilage running nearly the length of the body.
The skin is covered with large scales. The skeleton is soft, cartilaginous, and at a low stage of development, as in the lower Selachii and the earliest Ganoids. The chorda is completely retained, and surrounded by an unsegmented sheath. The two pairs of limbs are very simple fins of a primitive type, like those of the lowest Selachii.
But this unsegmented primary axial skeleton is soon replaced by the segmented secondary axial skeleton, which we know as the vertebral column. In the head-half of the embryo the skeletal plate remains a continuous, simple, undivided layer of tissue, and presently enlarges into a thin-walled capsule enclosing the brain, the primordial skull.
Word Of The Day