34 HISTOLOGY 



some in connection with the head of the spermatozoon in certain stages 

 (Fig. 24, C) but not in all. It is probable, according to Conklin, that 

 "the source of the cleavage centrosomes may differ in different animals, 

 or even in the same animal under different conditions." 



Later stages in the division or "cleavage" of the fertilized ovum into 

 two cells are shown in Fig. 24, H-J. The two groups of chromosomes 

 come together upon the spindle so that the full number, characteristic 

 of the species, is restored. Each chromosome then divides lengthwise, 

 and thus each daughter cell receives one-half of its chromosomes from 

 the male parent and one-half from the female parent. This is strikingly 

 evident when the eggs of the fish Fundulus, which have long rod-shaped 

 chromosomes, are fertilized with the sperm of Menidia, which has shorter 

 rods. Moenkhaus, who performed this experiment (Amer. Journ 

 Anat., 1904, vol. 3, pp. 29-64), states that the two kinds of chromosomes 

 remain grouped and bilaterally distributed on the spindles during the first 

 and second divisions of the fertilized ovum, but that later they become 

 gradually mingled. 



Important information in regard to the nature of fertilization has 

 been obtained by experiments upon unfertilized eggs. Changes in the 

 concentration or composition of the sea water in which the eggs of marine 

 animals have been placed, mechanical agitation, or, in the case of frogs' 

 eggs, puncturing the outer layer with a needle, have led to repeated cell 

 divisions. In this way embryos or larvae have been produced from 

 unfertilized eggs, and, in a few instances, adult animals. Loeb, who 

 has been a foremost investigator in this field, concludes that the sperma- 

 tozoon causes the development of the egg by carrying a substance into 

 it which liquefies the cortical layer of the egg, and thereby causes the 

 formation of a membrane. "This membrane formation, or rather the 

 modification of the surface of the egg which underlies the membrane 

 formation, starts the development." At the same time there is an accelera- 

 tion of the oxidations in the egg. "What remains unknown at present 

 is the way in which the destruction of the cortical layer of the egg accel- 

 erates the oxidations." 



For the physicist and chemist, the production of mitotic figures 

 and the process of fertilization, have been subjects of great interest, and 

 discussions of their significance will be found in various text-books of 

 physiology and biological chemistry. For further morphological details 

 the student is referred to "The Cell in Development and Inheritance," 

 by E. B. Wilson (2nd ed., New York, 1900) and to the chapters on "Die 

 Geschlechtszellen" and "Eireife, Befruchtung und Furchungsprozess," 

 by W. Waldeyer and R. Hertwig respectively, in vol. i of Hertwig's 

 "Handbuch der vergl. u. exp. Entwickelungslehre der Wirbeltiere," 

 (Jena, 1906). 



