Manual of physiology for students and practitioners / by H. Willoughby Lyle.

  • Lyle, H. Willoughby.
Date:
1911
Catalogue details

Licence: In copyright

Credit: Manual of physiology for students and practitioners / by H. Willoughby Lyle. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

    752/776 (page 728)
    At the outer end of the Fallopian tube spermatozoon and ovum meet, unite, and fertilisation occurs. The tail of the spermatozoon apjjears to become absorbed, the head remains as the male pronucleus. After the nucleus of the ovum has extended its two polar bodies, and in this Avay has got rid of half of its original number of chromosomes {maturaimi), it becomes the feviale pronucleus. The male pronucleus, which contains the same number of chromosomes as the female pronucleus, and the female pronucleus unite to form the segmentation nucleus {fertilisation), which now contains the normal number of chromosomes characteristic of the species. The ovum is also entered by the middle piece of the spermatozoon, which then induces the formation of a centro- some, which, after the completion of fertilisation, initiates the process of cell division. Cytoplasmic filaments become arranged in a star-like manner around the centrosome, forming the sperm-aster. The fertiUsed ovum, also called the oosperm or zygote, is thus produced, and forms the starting- point of that series of cell divisions which eventually result in the formation of a newly developed individual. SEGMENTATION. After fertilisation has become completed the oosperm undergoes cleavage, or segmentation, dividing first into two, and each of these again dividing into two, and so on, until there is formed a more or less globular mass of cells, or blastomeres. This mass of cells is the mulberry mass or morula. This process of segmentation of the ovum has never been observed in the human ovum, but it has been studied in the ova of the rabbit, in which the fertilised ovum divides into two; there is complete division of the cell, and the segmentation is therefore of the holoblastic type. There is, however, no reason to suppose that the segmentation of the human ovum materially differs from that of other mammals. In the morula a cavity soon appears, and the mass then forms a blastula, or blastocyst. The blastula is at first unilaminar except at one pole, where an inner group of irregular cells collects to form an inner cell