CHAPTER 8 

 THE EMBRYO 1 



After syngamy the zygote undergoes a period of rest during which 

 the large vacuoles originally present in the upper part of the cell 

 gradually disappear and the cytoplasm assumes a fairly homoge- 

 neous appearance. Vacuoles may appear once again when the cell 

 begins to grow in preparation for the first division, but these are 

 more or less uniformly distributed and are not restricted to any 

 special portion. 



The resting period of the zygote varies with different species and 

 is to some extent dependent on environmental conditions. In gen- 

 eral the primary endosperm nucleus divides first and the zygote 

 divides shortly afterwards. In Theobroma cacao (Cheesman, 1927) 

 the primary endosperm nucleus divides 4 to 5 days after fertilization 

 and the zygote 14 to 15 days after fertilization. In the fertile 

 banana "Rodoe Clamp" (White, 1928), Carya illinoensis (McKay, 

 1947), and Epidendrum prismatocarpum (Swamy, 1948) the zygote 

 remains undivided for about 6 weeks, and in Viscum alburn (Pisek, 

 1923) for about 8 weeks after fertilization. In Colchicum autumnale 

 (Heiman-Winawer, 1919) fertilization takes place in autumn and 

 endosperm nuclei are formed soon after, but the zygote remains 

 dormant for a period of 4 to 5 months during the winter. 



The shortest resting periods occur in the Compositae and Grami- 

 neae. In Crepis capillaris (Gerassimova, 1933), for example, the 

 primary endosperm nucleus undergoes its first division within 4 to 7 

 hours after pollination and its second division 1 to 3 hours later, 

 while the first division of the zygote takes place 5 to 10 hours after 

 pollination. In Oryza sativa (Noguchi, 1929) the first division of 

 the zygote occurs about 6 hours after fertilization and 18 hours later 

 the embryo consists of four to seven cells. After 4 days it begins to 

 differentiate into the various body regions and in 10 days it is 

 completely mature. 



The data on Hordeum distichon palmella (Pope, 1937) give the 



1 For a detailed treatment of the various modes of embryonal development met 

 with in the angiosperms, reference should be made to Soueges (1934a, 1937a, 1934- 

 1939). 



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