from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The nutritive substances or yolk in the cytoplasm of an ovum or other cell.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. The lifeless food matter in the cytoplasm of an ovum or a cell, as distinguished from the active or true protoplasm; yolk substance.
from the GNU version of the Collaborative International Dictionary of English
- n. The lifeless food matter in the cytoplasm of an ovum or a cell, as distinguished from the active or true protoplasm; yolk substance; yolk.
from The Century Dictionary and Cyclopedia
- n. In embryol., secondary, nutritive plasm, or food-yolk: a term applied by the younger Van Beneden to that portion of the yolk of an egg or ovum which furnishes food for the nourishment of the embryo, but does not enter directly into its formation or germination.
- n. Same as deuteroplasma.
In the mammalian ova the nutritive yolk or deutoplasm is small in amount and uniformly distributed throughout the cytoplasm; such ova undergo complete division during the process of segmentation, and are therefore termed holoblastic.
The yolk comprises (1) the cytoplasm of the ordinary animal cell with its spongioplasm and hyaloplasm; this is frequently termed the formative yolk; (2) the nutritive yolk or deutoplasm, which consists of numerous rounded granules of fatty and albuminoid substances imbedded in the cytoplasm.
Hence we distinguish in the ova two chief elements -- the active formative yelk (protoplasm) and the passive food-yelk (deutoplasm, wrongly spoken of as "the yelk").
The contractions of the active protoplasm, which effect this continual cleavage of the cells, meet a greater resistance in the lower vegetal half from the passive deutoplasm than in the upper animal half.
The chief part of the globular mass is formed by the nuclear yelk (deutoplasm), which is evenly distributed in the active protoplasm, and consists of numbers of fine yelk-granules.
In the yelk we must distinguish the active formative yelk (or protoplasm = first plasm) from the passive nutritive yelk (or deutoplasm = second plasm).
In these the contractility of the active protoplasm no longer suffices to break up the huge mass of the passive deutoplasm completely into cells; this is only possible in the upper or dorsal part, but not in the lower or ventral section.
While the protoplasm in the animal section of the ovum continues briskly to divide, multiplying the nuclei, the deutoplasm in the vegetal section remains more or less undivided; it is merely consumed as food by the forming cells.
It may, however, continue for some time (even after the gastrulation is more or less complete) in the sense that the vegetal cell-nuclei distributed in the deutoplasm slowly increase by cleavage; as each of them is surrounded by a small quantity of protoplasm, it may afterwards appropriate a portion of the food-yelk, and thus form a real "yelk-cell" (merocyte).
The clear protoplasm of the mature ovum is made so turbid by the numbers of dark granules of food-yelk or deutoplasm scattered in it that it is difficult to follow the process of fecundation and the behaviour of the two nuclei during it (Chapter