from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Embryology Undergoing partial cleavage. Used of a fertilized egg.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. that undergoes only partial cleavage
from the GNU version of the Collaborative International Dictionary of English
- adj. Consisting only in part of germinal matter; characterized by partial segmentation only
from The Century Dictionary and Cyclopedia
- In embryology, partially germinal: applied by Remak to those eggs in which there is much food-yolk which does not undergo segmentation or take part in germination: opposed to holoblastic. Birds, reptiles, most fishes, and most invertebrates have meroblastic eggs.
In the ova of birds, reptiles, and fishes where the nutritive yolk forms by far the larger portion of the egg, the cleavage is limited to the formative yolk, and is therefore only partial; such ova are termed meroblastic.
So far, the essential differences between the development of fowl and frog, the meroblastic segmentation, absence of a typical gastrula, and the primitive streak, seem comprehensible on the theory that such differences are due to the presence of an enormous amount of yolk.
Such a type of segmentation in which only part of the ovum segments is called meroblastic.
It is otherwise with the second chief group of ova, which he distinguished from these as meroblastic, or "partially-cleaving": to this class belong the familiar large eggs of birds and reptiles, and of most fishes.
This is comparatively easy in the small meroblastic ova which contain little nutritive yelk -- for instance, in the marine ova of a bony fish, the development of which I observed in 1875 at Ajaccio in Corsica.
But, besides these mesodermic cells of the "vascular layer" proper, other travelling cells, of which the origin and purport are still obscure, take part in the formation of blood in the meroblastic
This is the case with all meroblastic vertebrates, most fishes, the reptiles and birds, and the oviparous mammals (the monotremes).
The meroblastic ova are only found in the larger and more highly developed animals, and only in those whose embryo needs a longer time and richer nourishment within the foetal membranes.
Cleavage is meroblastic until approximately the 16-cell stage
The multiplicity or loss of the vitellogenin (vtg) gene family in vertebrates has been argued to have broad implications for the mode of reproduction (placental or non-placental), cleavage pattern (meroblastic or holoblastic) and character of the egg (pelagic or benthic).
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