Gametogenesis, Fertilization and Parthenogenesis 



199 



gen) upon fertilization in the sea urchin 

 Paracentrotus lividus. Expressed as glucose 

 the breakdown in the first 10 minutes after 

 fertilization averages 4.7 mg. per 100 mg. of 

 egg nitrogen (= 2.8 cc. of eggs = 5 X 10^ 

 eggs = 0.7 gm. dry weight of eggs), whereas 

 during the next 3 hours it is less than 3 per 

 cent of that value (see Fig. 65). If completely 

 oxidized this breakdown would be more than 

 ample to account for the oxygen uptake fol- 

 lowing fertilization. The available data on 



FUSION OF PRONUCLEI 



Very little is known concerning the factors 

 responsible for the union of the pronuclei 

 within the egg. Here again attractive in- 

 fluences have been proposed. However, it is 

 known that the egg nucleus in artificially 

 activated eggs can migrate in the same 

 manner as if it were fertilized. Similarly, 

 upon fertilization of non-nucleated eggs the 

 sperm nucleus attains a normal position for 



Fig. 66. Behavior of pronuclei in centrifuged eggs of the sand dollar Dendraster. A, Two successive stages 

 (at interval of I/2 hour) of an egg in which a sperm entered at centripetal end, fused with egg pronucleus 

 and the fusion nucleus moved into large centrifugal part of the egg. B, Two successive stages (at interval of 

 2 hours) of an egg in which a sperm entered at the centrifugal end, failed to unite with egg pronucleus and 

 proceeded to segment the centrifugal end. e. Egg pronucleus; 5, sperm pronucleus; es, fusion nucleus; o, oil 

 cap. (After Moore, '37.) 



the respiratory quotients, however, show a 

 decrease from about unity in the unfertilized 

 sea urchin egg (Borei, '33) to values vari- 

 ously reported as ranging from 0.64 to 0.85 

 (Ephrussi, '33; Borei, '33; Laser and Roths- 

 child, '39; Ohman, '40) after fertilization. 

 From analyses of lipids Ohman ('44) sug- 

 gests that these represent the chief energy 

 source, whereas from analysis of ammonia 

 liberation Hutchens et al. ('42) suggest oxi- 

 dation of protein. These various investiga- 

 tions have evidently all been carefully per- 

 formed, but it is evidently still necesary to 

 determine to what extent the divergent re- 

 sults are due to technical difficulties and to 

 species differences. Most investigators, how- 

 ever, are in accord with the view that differ- 

 ent substrates are utilized before and after 

 fertilization (see Brachet, '47, for further 

 details). 



cleavage of the cell. The fact that egg and 

 sperm pronuclei may meet at some distance 

 from the ultimate position of the fusion 

 nucleus does not imply attraction, since it 

 has not been shown that they are diverted 

 from the path that they would take inde- 

 pendently. Wilson ('25) and Morgan ('27) 

 have reviewed the early work on this subject 

 and little has been added since that time. 

 Moore ('37) has studied the movements 

 of the pronuclei in echinoid eggs that had 

 been drawn out into flask-shaped form as a 

 result of high speed centrifugation. He found 

 that when the sperm entered the end of the 

 egg containing the egg nucleus it fused with 

 the latter and the fusion nucleus moved 

 through the "neck" into the larger mass of 

 cytoplasm (Fig. 66A). When the sperm 

 entered the opposite end (Fig. 665) it re- 

 mained there and proceeded to segment that 



