FERTILIZATION. 41 



tion — "a conclusion suggested by the experience of practical breeders of plants and animals." 

 Weismann himself holds that the need of fertilization is a secondary matter, but that the 

 admixture of different germ plasms insures the mingling and renewal of variations. 



Spencer and Darwin, on the other hand, believe that although crossing among animals or 

 plants may lead to variability within certain limits, it tends in the long run to hold in check 

 any wide digression from a norm and hold the species true to type. 



PRACTICAL SUGGESTIONS. 



Fertilization of the ovum may be observed in some of the Invertebrates and makes a 

 wonderfully interesting process to observe under the microscope. In the sea-urchin, for 

 example, whose ova are relatively small and transparent, the steps may be followed in the 

 living objects. The mature ova are removed from the animal and placed in a drop of sea- 

 water on a slide; then a drop of seminal fluid mixed with sea-water is added. A cover-glass 

 is gently applied, and it is best to place a thin bit of glass under one edge to prevent crushing. 

 The following phenomena may be seen under a fairly high power lens: In less than a minute 

 a vast number of the spermatozoa are clustered around each egg. Under the most favorable 

 conditions one of these spermatozoa may soon be seen with its head attached to the ovum. 

 The head penetrates deeper into the cytoplasm, the tail becomes motionless and finally 

 invisible. The egg nucleus and sperm nucleus then seem to exert a mutual attraction and 

 move toward each other. They finally come in contact and fuse, the product of their fusion 

 being the segmentation nucleus. The whole process of fertilization has not occupied more 

 than ten minutes. 



In Ascaris the behavior of the sperm nucleus during the maturation of the ovum may be 

 inferred from the different stages (see "Practical Suggestions," p. 33). In this particular 

 animal the two pronuclei do not actually fuse but return to the resting stage within the ovum 

 and then, when the first cleavage is about to occur, break up into their respective chromo- 

 somes. 



References for Further Study. 



Conklin, E. G.: The Embryology of Crepidula. Jour, of Morphol, Vol. XIII, 1897. 



Harper, E. H. : The Fertilization and Early Development of the Pigeon's Egg. Am. 

 Jour, of Anat., Vol. Ill, No. 4, 1904. 



, Hertwig, R.: Eireife, Befruchtung u. Furchungsprozess. In Hertwig's Handbuch d. 

 vergleich. u. experiment. Entwickelungslehre der Wirbeltiere, Bd. I, Teil I, 1903. 



King, H. D.: The Maturation and Fertilization of the Egg of Bufo lentiginosus. Jour, 

 of Morphol., Vol. XVII, 1901. 



Sobotta, J.: Die Befruchtung u. Furchung des Eies der Maus. Arch. f. mik. Anat., Bd. 

 XLV, 1895. 



Wilson, E. B.: The Cell in Development and Inheritance. 2d Ed., 1900. 



