GENERAL ZOOLOGY 



Polocvtes -Fertilization membrane 



Female 

 'pronucleus 



Perivitelline space 



Fig. 5.12. Early development after artificial parthenogenesis in 

 Tkalassema, a worm-like invertebrate. A, meiosis has occurred within 

 a fertilization membrane, the formation of which was conditioned by 

 a short treatment with dilute acid. B, the first cleavage spindle; 

 only the haploid number of chromosomes is present. (From G. 

 Lefevre, 1907, Journal of Experimental ^oology, vol. 4.) 



spermatozoa are shed over the egg mass in the water. FertiUzation takes 

 place in the water, outside the body. In vertebrates that Hve on the land, 

 fertihzation is internal. Copulation occurs, and spermatozoa are introduced 

 into the reproductive tract of the female, where they can swim in the seminal 

 fluid and in the liquid filling the female ducts. The spermatozoa pass into 

 the Fallopian tubes and meet the eggs as they are ovulated. In most reptiles 

 and birds additional food material in the form of albumen is secreted about 

 the fertilized egg, or zygote, as it passes down the oviduct. A shell, which 

 prevents drying and serves as a protection for the developing individual, is 

 added before the egg is laid or passed out of the female's body to develop 

 (Fig. 5.4). The mammalian zygote is retained and nourished in the uterine 

 portion of the oviducts during the developmental period. 



Usually only one spermatozoon penetrates an egg cell, but in some verte- 

 brates, such as the birds, polyspermy is a normal occurrence; that is, several 

 spermatozoa enter each egg. However, the nucleus of only one of these 

 spermatozoa fuses with the egg nucleus in amphimixis. In 1875, Fol observed 

 that, as soon as one spermatozoon had reached the egg, a membrane, known 

 as the fertilization membrane, becomes separated from the surface of the egg, 

 leaving a perivitelline space around the egg (Figs. 5.9 and 5.1 1). The fertiliza- 

 tion membrane was believed by many students to be a device to prevent 

 polyspermy but is now recognized as a by-product of activation. Such a 

 membrane is formed when eggs are artificially stimulated to develop (Fig. 

 5.12). The spermatozoon swims actively until it comes in contact with the 

 surface of the egg; the egg then engulfs the spermatozoon after first sending 

 out a minute projection, the entrance cone, in which it becomes embedded (Fig. 



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