. A textbook of invertebrate morphology [microform]. Invertebrates; Morphology (Animals); Invertébrés; Morphologie (Animaux). I 46 INVERTEBRATE MORPHOLOGY. i ⢠I I I the oviduct, leading to the exterior. At the tip of each tube the primitive germ-cells (Fig. 20, g) are located, and lower down ova (o) in various stages of development towards matu- rity are to be found, each surrounded by a number of small undeveloped germ-cells, known as follicle-cells {/), whose func- tion it is to transfer food-yolk {y) to the growing ovum. As the latter approaches maturity the follicle-cells secrete aroun
. A textbook of invertebrate morphology [microform]. Invertebrates; Morphology (Animals); Invertébrés; Morphologie (Animaux). I 46 INVERTEBRATE MORPHOLOGY. i ⢠I I I the oviduct, leading to the exterior. At the tip of each tube the primitive germ-cells (Fig. 20, g) are located, and lower down ova (o) in various stages of development towards matu- rity are to be found, each surrounded by a number of small undeveloped germ-cells, known as follicle-cells {/), whose func- tion it is to transfer food-yolk {y) to the growing ovum. As the latter approaches maturity the follicle-cells secrete around it a thick, sometimes highly sculptured shell and finally degenerate. As a rule, conjugation with a spermatozoon, fertiliza- tion, is necessary as an antecedent to further development. Before this takes place, however, certain modifications of the ovum are necessary, the pheno-^^ona which accompany them being known as the maturatio, / the ovum. In this process. A B Fig. 21.âDiaohams illustuatino the ^Iatukation of tiir Ovum. A = foriuutioii of the Hist polar globule (pg). B â fonnatiou of llie second polar globule aud eutiauce of tlie sperm-nucleus («/>)â (I'ig. 21, A) the nucleus approaches the surface of the ovum aud there undergoes a karyokiuetic division which is peculiar in that in the equatorial-plate stage twice as many chromo- somes are formed as are typical for the species. These do not undergo longitudinal division, and by the karyokinesis their number is reduced to the typical number, a small cell, the polar globule (pg), being separated from the ovum with lialf the chromosomes, while the others are retained witliin the ovum. Tht nucleus of the ovum, instead of now returning to the resting stage, divides again (Fig. 21, B), a second polar globule being formed and receiving half the chromosomes. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of
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Keywords: ., bookcentury1800, bookdecade1890, bookpublishernewyo, bookyear1894
