. Text-book of embryology. Embryology. INTEODUCTION 11 dula plana, a species in which the processes of maturation and fertilization have been worlced out in great detail by Conklin (1902). The centrosomes of the first cleavage spindle are here stated to^be derived from the division of the sperm centrosome. It reasonably IS therefore certain that m each daughter. all cases ^jL cell re- ceives a half of each male and female chromosome. It is by no means always true that the spermatozoon can only enter the egg after the formation of both polar bodies. In many cases it enters the egg whilst
. Text-book of embryology. Embryology. INTEODUCTION 11 dula plana, a species in which the processes of maturation and fertilization have been worlced out in great detail by Conklin (1902). The centrosomes of the first cleavage spindle are here stated to^be derived from the division of the sperm centrosome. It reasonably IS therefore certain that m each daughter. all cases ^jL cell re- ceives a half of each male and female chromosome. It is by no means always true that the spermatozoon can only enter the egg after the formation of both polar bodies. In many cases it enters the egg whilst it is stiU an oocyte of the first order, and even before the nuclear mem- brane has been dissolved and the germinal spot has disappeared. This is true of the eggs of many Annelida and MoUusca. In other cases, such as in some Ascidians, the first maturation spindle is formed before the spermatozoon enters, but the first maturation division is not completed till the spermatozoon is inside the egg. Finally, in Binophilus according to Shearer (1912), the spermatozoon enters the oogonium and remains passive during the growth and maturation of the germ cell. If the eggs are stale, if they have been shed too long from the ovary before being fertihzed, then more than one spermatozoon can enter them, and an extra centrosome is thus introduced, between which and one or both of the centrosomes resulting from the division of the centrosome of that sperm which has actually effected fertihza- tion, extra achromatic fibres can be developed and irregular division Fig. 4.—Two stages in the first division of the fertilized egg of Crepidulaplana. (After Conklin.) A, the first cleavage spindle ; female cliromosomes above separated by an interval from malejCbromosome below. B, the division of the zygote nucleus is complete. /, Female chromo- somes ; m, male chromosomes ; pi (in A), first polar body, (in B), products of division of iirst polar body; p'^, second polar Please note that
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