Gametogenesis, Fertilization and Parthenogenesis 



177 



germ cells. These and related experiments 

 are interpreted to mean that the dorsocaudal 

 entoderm has a germ cell-inducing in- 

 fluence on certain predisposed cells of the 

 presumptive lateral plate mesoderm, with 

 which it comes in contact at the end of 

 gastrulation and during neurulation. On the 

 basis of early determination and segregation 

 this implies that the germ cells are at first 

 simply endowed with the capacity (compe- 

 tence) to react to such inductive stimulus. 

 The inductive stimulus is apparently not pro- 

 vided by other organs such as the notochord 

 and Wolffian duct. That the reactive cells 

 are not simply ordinary lateral plate 

 cells is illustrated by the absence of germ 

 cells in embryos in which the lateral plate 

 was partially reformed (after removal of its 

 presumptive material) by regulation from 

 more dorsal mesoderm. Nieuwkoop also 

 finds, as Humphrey had demonstrated ear- 

 lier, that the germ cells do not cross over 

 from one side of the embryo to the other. 

 Concerning the formation of a second gen- 

 eration of germ cells in the genital ridge, 

 Nieuwkoop finds no indication of this in 

 any of the experimental animals in which 

 the presumptive germ-cell material was 

 removed. This is very well shown, too, in 

 his heteroplastic transplantations in which 

 the host germ ridge may contain exclusively 

 donor germ cells. So the possibility that the 

 presence of primary primordial germ cells 

 may induce genital ridge cells to form 

 secondary germ cells is also ruled out. 



GROWTH OF THE OOCYTE 



Cytological Investigations. There has been a 

 considerable amount of cytological investiga- 

 tion into the manner of yolk formation, cen- 

 tering primarily upon the possible role of 

 various cytoplasmic and nuclear structures, 

 such as the mitochondria, Golgi bodies, 

 chromidia, nucleoli and hyaloplasm. Vari- 

 ous workers have implicated one or another, 

 or some combination, of these structures in 

 the process and there is considerable differ- 

 ence of opinion even when the same material 

 is studied. (See, for example Harvey, '29, 

 for references to four investigations, and 

 four different interpretations, of yolk forma- 

 tion in the centipede Lithobius.) The earlier 

 work has been reviewed by Wilson ('25) and 

 MacBride and Hewer ('31). For some of the 

 more recent work reference may be made to 

 the studies of Subramaniam and Aiyar ('36) 

 on sea urchins, of Narain ('37) and Singh 

 and Boyle ('38) on fish, of Beams and King 



('38) on the guinea pig, of Worley ('44, '46) 

 on mussels and of Bretschneider ('46) on 

 snails. No general conclusion concerning 

 the mechanism of yolk formation appears to 

 be, as yet, firmly established. The only 

 point on which there seems to be fair agree- 

 ment among various workers is that the fatty 

 yolk is formed in association with Golgi 

 bodies (see Worley, '44; Bretschneider, '46). 

 Two techniques that hold promise of effec- 

 tive use in further research in this field are 

 centrifugation, employed by Beams and 

 King ('38) and Singh and Boyle ('38), and 

 vital staining, employed by Worley ('44). 



Table 9. Relative Radiophosphorus (P^^) Con- 

 tent of the Phosphatide-Phosphorus 

 Extracted from Various Organs of a 

 Laying Hen 5 Hours after a Subcu- 

 taneous Injection of 10 mg. of La- 

 helled Sodium Phosphate [from 

 Hevesy and Hahn, '38) 



P'2 CONTENT PER MG. P RELATIVE 

 ORGAN TO THE p'^ PER MG. INORGANIC P 

 OF THE PLASMA TAKEN AS 100 



Chemical Investigations. In addition to the 

 more strictly cytological investigations there 

 have also been many contributions of a 

 histochemical type on the changes that 

 occur during oogenesis. Extensive accounts 

 of these are given by Needham ('31, '42), 

 Marza ('38), and Brachet ('47), along with 

 summaries of work done by direct chemical 

 methods. In some of the recent work in this 

 field, as in others, the use of radioactive 

 tracers has provided valuable information. 

 In particular this technique has contributed 

 information concerning the question whether 

 various constituents of the yolk are synthe- 

 sized within the oocyte or in some other 

 tissue of the body. A brief account of the 

 pertinent experiments is presented in the 

 following section. 



Tracer Experiments. The experiments re- 

 lating to this have been done with radio- 

 active phosphorus (P^^) as a tracer for the 

 formation of phosphatides (lecithin and 

 cephalin) and phosphoprotein (vitellin) in 

 eggs of the hen by Hevesy and Hahn ('38), 



