BIOLOGY OF EGGS AND IMPLANTATION 



811 



by radioautographic techniques. Ovarian 

 ova and the blastocysts recovered from the 

 cornua showed active protein synthesis. Sim- 

 ilar synthesis was noted in the early fertili- 

 zation stages. However, eggs in the 2-cell 

 through the morula stages contained no 

 demonstrable S^^ methionine. From these 

 observations one would conclude that there 

 is a basic difference in the metabolism of 

 tubal and cornual ova, and again raises 

 the question of the importance of the en- 

 vironmental fluids in providing materials 

 necessary for the growth and development 

 of the eggs. 



Earlier investigators directed attention to 

 the fact that in many mammalian eggs the 

 deutoplasm is arranged in such a way as to 

 exhibit an obvious polarity. Such polarity 

 was described particularly for the eggs of 

 the guinea pig by Lams (1913) and is con- 

 spicuous in a newly ovulated egg found in 

 section by Myers, Young and Dempsey 

 (1936). Such a polarity has been observed 

 also in eggs of the cat (Van der Stricht, 

 1911), bat (Van Beneden, 1911), dog (Van 

 der Stricht, 1923), and ferret (Hamilton, 

 1934). 



Attention has recently been redirected to 

 the fact that the mammalian egg may have 

 a specific cytologic organization which is 

 important in establishing its symmetry and 

 polarity. This pattern of symmetry is based 

 on the crescentic distribution of a primary 

 basophilia and the localization of the mito- 

 chondria. The significance of the cytoplas- 

 mic organization in relation to the morpho- 

 genetic pattern in the mammalian egg must 

 await the elaboration of new techniques of 

 experimental embryology which can be ap- 

 plied to mammalian material ( Jones-Seaton, 

 1949; Dalcq, 1951, 1955; Austin and Bishop, 

 1959). 



There are striking species differences in 

 the amount and distribution of yolk material 

 within the cytoplasm of living mammalian 

 eggs. In the eggs of the horse, cow, dog, and 

 mink the cytoplasm is so filled with fatty 

 and highly refractile droplets that the vitel- 

 lus under phase microscopy appears as a 

 dark mass obscuring the nucleus (Squier, 

 1932; Enders, 1938; Hamilton and Day, 

 1945; Hamilton and Laing, 1946). In living 

 eggs of the monkey, rat, mouse, rabbit, ham- 

 ster, and goat the yolk granules are finely 



divided and vmiformly distributed; thus the 

 various nuclear changes occurring during 

 meiosis and fertilization are more readily 

 visible (Long, 1912; Lewis and Gregory, 

 1929; Lewis and Hartman, 1941; Amoroso, 

 Griffiths and Hamilton, 1942; Samuel and 

 Hamilton, 1942; Austin and Smiles, 1948; 

 Blandau and Odor, 1952). The ooplasm of 

 human and guinea pig eggs is of interme- 

 diate density when compared to the two 

 groujis mentioned above (Squier, 1932; 

 Hamilton, 1944). 



The mature mammalian egg is a cell of 

 extraordinary size, and even the smallest 

 (field vole, 60 //,) is large when compared 

 with any of the somatic cells within its en- 

 vironment. It is remarkable that through- 

 out the eutheria there should be so little re- 

 lationship between the size of the adult 

 animal and the volume of the egg (Hartman, 

 1929). Data on the apparent sizes of the 

 vitelli of living eggs of various animals are 

 summarized in Table 14.1. The need for 

 more accurate measurements on the diam- 

 eters and volumes of the living eggs of mam- 

 mals still exists. 



C. EGG MEMBRANES 



1. The Zona Pellucida 



The zona pellucida is usually classified as 

 a secondary egg membrane. It is believed to 

 be a product of the primary layer of follicu- 

 lar cells which surround the oocytes in the 

 ovary (Corner, 1928a). Under the light mi- 

 croscope the fresh zona pellucida appears as 

 a more or less homogeneous membrane with 

 a somewhat irregular surface, the amount of 

 irregularity depending upon the species. As 

 mentioned earlier the immature mammalian 

 oocyte is surrounded by a single layer of 

 cuboidal "follicle cells" whose plasma mem- 

 branes are in intimate contact with the 

 vitelline membrane. This relationship is par- 

 tially altered in the growing egg by the 

 gradual deposition of a mucopolysaccharide 

 membrane which when fully formed consti- 

 tutes the zona pellucida. At first the zona 

 pellucida appears in irregular patches and 

 in the form of an homogeneous secretion 

 (Fig. 14.7). Slender microvilli which extend 

 from the surface of the vitelline membrane 

 are embedded in the zona. Short, blunt cel- 

 lular processes also arise from the granulosa 



