2 THE EGGS OF MAMMALS 



Bischoff, 1842). Following Barry's (1839) initial observation 

 of cytoplasmic cleavage there ensued a long series of ob- 

 servations on the developmental history of fertilized eggs. 

 Attention gradually shifted from living eggs to fixed speci- 

 mens, chiefly employed for the determination of the exact 

 cytology of fertilization and the histological changes occur- 

 ring during differentiation. This resulted in the publication 

 of numerous detailed descriptions of the early embryology 

 in various classes of mammals (Bischoff, 1845, 1852, 1854; 

 Bonnet, 1884, 1891; Caldwell, 1887; Hartman, 1916, 1919; 

 Heape, 1883, 1886; Hensen, 1876; Hill, 1910, 1918; Hill and 

 Tribe, 1924; Huber, 1915; Hubrecht, 1912; Jenkinson, 1900, 

 1913; Keibel, 1888, 1894, 1899, 1901, 1902; Lams and 

 Doorme, 1908; Lams, 1910, 1913, 1924; Melissinos, 1907; 

 Minot, 1889; van Oordt, 1921; Reichert, 1861; Rein, 1883; 

 Robinson, 1892; Sakurai, 1906; Selenka, 1883, 1884, 1887; 

 Sobotta, 1893, 1895; Tafani, 1889; Van Beneden, 1875, 

 1880, 1899, 1911, 1912; Van Beneden and Julin, 1880; Weil, 

 1873; Wilson and Hill, 1907). The hving egg was neglected 

 presumably because no technique was developed for pre- 

 serving it intact in vitro long enough for any extensive 

 experimentation to be performed. Nor did the possibility 

 of experimental manipulation of ova in vivo receive more 

 than passing attention (see Grusdew, 1896; Novak and 

 Eisinger, 1923). 



Since the pubhcation of Stockard and Papanicolou's (1917) 

 and Long and Evans' (1922) exhaustive accounts of the 

 oestrus cycle of the guinea pig and rat respectively, a new 

 era in the study of sexual physiology has been initiated. 

 Enormous strides have been made in the discovery and 

 purification of the hormones regulating the activities of the 

 genital tracts of mammals. The ovarian control of the various 

 phases of the sex cycles in the female has received exhaustive 

 attention, and the control of gonad function by the anterior 

 pituitary has been investigated in detail. Despite the enor- 

 mous accumulation of data on the endocrine regulation of 

 the ovarian and oviduct environment of ova, the ova them- 



