. Outlines of zoology. Zoology. FER TILISA TION. 61 it required to be fed by the ovum. Even after it was recognised that both kinds of reproductive elements were essential, many thought that their actual contact was un- necessary, that fertilisation might be effected by an aura seminalis. Though spermatozoa were distinctly seen by Hamm and Leeuwenhoek in 1679, their actual union with ova was not observed till 1843, when Martin Barry detected it in the cs Fig. 28.—Fertilisation in Ascaris megalocephala, —After Boveri. Spermatozoon (sp.) entering ovum, which contains reduced nucleus (JV
. Outlines of zoology. Zoology. FER TILISA TION. 61 it required to be fed by the ovum. Even after it was recognised that both kinds of reproductive elements were essential, many thought that their actual contact was un- necessary, that fertilisation might be effected by an aura seminalis. Though spermatozoa were distinctly seen by Hamm and Leeuwenhoek in 1679, their actual union with ova was not observed till 1843, when Martin Barry detected it in the cs Fig. 28.—Fertilisation in Ascaris megalocephala, —After Boveri. Spermatozoon (sp.) entering ovum, which contains reduced nucleus (JV), having given off two polar bodies ( i and 2). Sperm nucleus (the upper), and ovum nucleus (N), each with two chromatin elements or idants, and with centrosom^s (). Centrosomes () with " archoplasmic " threads radiating outwards in part to the chromosomes of the two approximated nuclei. Segmentation spindle before first cleavage. Of the many facts which we now know about fertilisation, the following are the most important:— (1) Apart from the occurrence of parthenogenesis in a few of the lower animals, an ovum begins to divide only after a spermatozoon has united with it. After one sper- matozoon has entered the ovum, the latter ceases to be receptive, and other spermatozoa are excluded. If, as rarely happens, several spermatozoa effect an entrance into the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Thomson, J. Arthur (John Arthur), 1861-1933. Edinburgh, Glasgow and London, H. Frowde and Hodder & Stoughton
Size: 1679px × 1488px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
