yiS NITROGEN METABOLISM AND GROWTH 9 



maturation of the ovum when incomplete, and for initiation of the cleavage 

 process with attendant physiological changes, little is known of the chemistry 

 involved. Lundblad (1950) reports strong but short-lasting proteolytic activity 

 at fertilization in three species of sea urchin, disappearing after 30 min. After 

 7 h., proteolytic activity is resumed and continues to increase up to 30 h. Lund- 

 blad's observation may be correlated with that of Riinnstrom (1949), who 

 describes disintegration of cortical granules at the time of activation of the egg. 



Because of the importance of the cortical and nutritional endowment of the 

 egg, some attention is due the source of its dowry. Panijel (1950, 1951) has 

 followed the chemical development of the egg of the frog, Rana temporaria, in 

 which he describes the early accumulation of RNA, glycogen and mitochondria 

 and the final addition of yolk platelets. Grant (1953) has summarized Panijel's 

 findings (Fig. i). The platelets are predominantly of two sizes (Panijel, 1950): 

 large (35 [i) which consist mainly of protein and lipid, and small (2.0 \i) containing 

 relatively more RNA and enzymes. Similarly a mammalian egg has been followed 

 through oogenesis (Alfert, 1950); he concludes that, as far as the evidence goes, 

 the ripe egg contains a store of proteins which is simply parceled out to the cells 

 and nuclei resulting from the first few cleavage divisions. 



Recently, Faure-Fremiet, Ebel and Colas (1954) studied the accumulation of 

 protein granules in Parascaris eqiiorum, their migration to the surface and dispersal 

 in the perivitelline space. They conclude that they constitute a source of nourish- 

 ment for the developing embryo. This view differs from the observations of 

 Panijel (1951) who asserts that the sperm contributes nutritional material of a 

 protein nature; however, he does not extend this conclusion to other invertebrates. 



The chemical growth of the hen's egg has been more extensively studied both 

 by chemical analysis and by experiment. Romanoff and Romanoff (1949) 

 tabulate the average quantities of several groups of constituents of the egg: 

 protein content is approximately 12.8% of the total dry matter which is 26.4% 

 of the entire egg content. At the time of hatching, the oocyte cytoplasm consists 

 of water (86%) and protein, and growth of the oocyte consists principally of 

 addition of these substances. Marza and Marza (1935) correlate the transfor- 

 mation of the foUicular epithelium and of the vitelline membrane with yolk 

 formation. They describe three growth phases characterized by the type of 

 material included in the yolk: (j) formation of a peripheral fatty layer (pre- 

 hatching) ; (2) addition of protein and phosphatides (from hatching to about two 

 months of age) ; (j) deposition of phospholipids and yolk protein, principally 

 ovovitellin. 



In the chicken egg, it is of some interest to note that prematurity of ovulation, 

 induced by desiccated male pituitary, does not impede embryonic development. 

 Neher and Fraps (1946) found fertilizable 47 out of 53 prematurely ovulated eggs; 

 of the 47, 38 hatched. When the prematurity was 22 h. or less, all hatched; but 

 30 h. prematurity (2 cases) apparently prevented hatching, but both embryos 

 lived for a substantial part of the incubation period (one near hatching). Perhaps 

 these data merely confirm the expected, for the last 30 h. of the period of egg 

 formation would not alter greatly the egg contents. 



The work of Hevesy (1947) and of Chargaff (1942) with 32P establishes quite 



