GENERAL BIOLOGY OF EGGS 



rabbit, and by BischorT( 1842a, b, 1845, 1852, 1854b, 1863) in several 

 species. Through the work of Schwann (1839) and Gegenbaucr 

 (1861; cited by Nordenskiold, 1928), the ovarian egg was shown 

 to be a single cell. By the middle of the century, it was known that 



Fig. 5 



Stages of cleavage in the dog egg (BischofF, 1845). Numerous 

 spermatozoa are shown attached to the zona pellucida. 



the mammalian egg consisted of a cytoplasmic mass or vitellus, 

 containing a nucleus which was termed the germinal vesicle, and 

 surrounded by a thick transparent membrane, the zona pellucida. 

 The earliest intimations that spermatozoa enter eggs were provided 

 by Barry (1843), BischorT (1854a) and Meissner (1855) in the rabbit, 

 Nelson (1851) in Ascaris, and Newport (1853) in the frog, but the 

 first worthwhile descriptions of fertilization are those of Van 

 Beneden (1875) in the rabbit, Hertwig (1876) and Fol (1877, 1879) 

 in sea urchin and starfish and Van Beneden and Julin (1880) in 

 bats. From these observations, in the main, the realization came 

 that fertilization involved the union of egg and sperm nuclei and 

 represented therefore the cytological mechanism underlying bi- 

 parental inheritance. Before the close of the century, Sobotta 

 (1895) published his classical account of maturation, fertilization and 

 cleavage in the mouse egg, based upon one of the earliest applica- 

 tions of the histological technique to the study of eggs. The last 



