. Outlines of zoology. vity sacrifice finds its justification inoccupied by liquor foUiculi. the increased specialisation of the accordance with the effect of the presence of yolk as noted above,we find that segmentation is total (holoblastic) in the ova of the lam-prey, the sturgeon, Ceratodus, Amphibians, and all Mammals except theMonotremes. In the ova of Elasmobranchs, Teleosteans, Reptiles,Birds, and Monotremes, the activity of the protoplasm is not sufficientto overcome the inertia of the yolk, and segmentation is partial(meroblastic). Similarly we find that a gastrula is fo
. Outlines of zoology. vity sacrifice finds its justification inoccupied by liquor foUiculi. the increased specialisation of the accordance with the effect of the presence of yolk as noted above,we find that segmentation is total (holoblastic) in the ova of the lam-prey, the sturgeon, Ceratodus, Amphibians, and all Mammals except theMonotremes. In the ova of Elasmobranchs, Teleosteans, Reptiles,Birds, and Monotremes, the activity of the protoplasm is not sufficientto overcome the inertia of the yolk, and segmentation is partial(meroblastic). Similarly we find that a gastrula is formed, in part at least, by dis-tinct invagination in the development of the lamprey, the sturgeon,and Amphibians (though the occurrence of invagination has been deniedfor the frog); it is more modified in Teleosteans and Elasmobranchs,whose ova have more yolk ; it is much disguised in Sauropsida andMammals. Most Vertebrates lay eggs in which the young are hatchedoutside of the body, and to all these forms the term ovi-. REPRODUCTIVE SYSTEM. 515 parous is applied. In some sharks, a few Teleosteans, sometailed Amphibians, a few lizards and snakes, the young arehatched before they leave the body of the mother. Tothese cases the awkward term ovo-viviparous is applied, butthere is no real distinction between this mode of birth andthat called oviparous, and both may occur in one animal•( in the grass-snake) in different conditions. In theplacental Mammals there is a close organic connectionbetween the unborn young and the mother, and theparturition in this case is usually called viviparous. But allthe three terms are bad. CHAPTER XXI PHYLUM CHORDATASUB-PHYLUM CRANIATA Class CYCLOSTOMATA{Synonym, Marsipobranchii) The hag (Myxine), the lamprey (Petromyzon), and a fewothers hke them, differ in so many ways from Fishes, thatthey must be ranked in a distinct class. They represent anarchaic type, whose interest has been enhanced by thediscovery of Palceospondylus in the Old Red Sands
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
