The frog : an introduction to anatomy, histology, and embryology / by the late A. Milnes Marshall ; edited by F.W. Gamble.

  • Marshall, A. Milnes (Arthur Milnes), 1852-1893.
Date:
1912
Catalogue details

Licence: In copyright

Credit: The frog : an introduction to anatomy, histology, and embryology / by the late A. Milnes Marshall ; edited by F.W. Gamble. Source: Wellcome Collection.

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    As this elongates it carries with it an extremely delicate layer of cytoplasm. The thread becomes contractile, and in this way the tail of the spermatozoon is formed, the centrosome remaining at the root of the tail as the middle piece. The head of the spermatozoon arises chiefly from the condensed nucleus of the spermatid, and possesses a thin covering of cytoplasm. Examine prejyared sections of the testis of the newt, frog, or rat. C. Formation of the Zygote. If a mature ovum, as described above, be fertilised by a ripe spermatozoon, the following changes take place. By the aid of its vibratile tail the spermatozoon attaches itself to the ovum. The head and middle piece pass into the cytoplasm of the egg, leaving the tail behind. The ovum now contains two nuclei and two centrosomes, belonging to the spermatozoon and ovum respectively. The two centrosomes approach one another, and between them is formed a spindle upon which the chromosomes of the egg-nucleus and those of the sperm-nucleus are dis- tributed. In some animals the paternal and maternal chromo- somes are distinct, and in such cases it can be seen that each set divides, and the four groups go to opposite poles of the spindle—two to each. The whole cell, or zygote, now divides, so that each daughter-cell receives an equal number of both paternal and maternal chromosomes. These being reduced by the changes described on pp. 168-9, now by addition amount to the full number characteristic of the species.