248 THE BIOLOGY OF BIRDS 



degenerate. But some of them reach a definite locus — the 

 germinative epithelium. 



(2) A special portion of mesoderm gives rise to the " uro- 

 genital connections," the " rete ovarii " in the case of the 

 ovary. Of similar origin is the connective-tissue framework. 



(3) The germinative epithelium gives rise by proliferation 

 to buds or strands, the first set transient, the second set 

 including the secondary gonocytes. 



Two points of general biological interest must be noticed. 

 (a) The appearance of primordial sex-cells on the second day 

 of embryonic development illustrates the early segregation of 

 reproductive cells, and is in perfect harmony with Weismann's 

 doctrine of the continuity of the germ-plasm. Yet the 

 connection between these primordial sex-cells and the 

 secondary gonocytes which form the definitive eggs and 

 sperms is not very clear. It may be, as Firket suggests, 

 that the primordial sex-cells have only a historical interest, 

 that they are the vestiges of an old-fashioned method of 

 gonocyte-production which persists in Cyclostomes and 

 Lancelets. (b) Among the possible ova produced from the 

 secondary gonocytes in the cortical strands of the ovary there 

 is a sort of struggle for existence, and only a small percentage 

 attain to maturity. If there is any discriminateness in the 

 intra-ovarian elimination that goes on, it may be of some 

 importance in evolution — securing the survival of certain 

 types of ovum-constitution. 



§ 2. Yolk-forming 



The young egg-cells or oocytes in the ovary accumulate 

 nutritive reserves of yolk, which is brought to them by the 

 blood and the lymph in the form of dissolved proteins and 

 fatty acids. Acting on this material, the immature egg-cell 

 lays it down within itself in the form of minute " yolk plates." 

 The increase in the size of the immature egg comes about 

 very rapidly in birds, as also in reptiles. Riddle (1916) 

 notes that when the oocyte or young ovum of the fowl reaches 

 a diameter of 6 mm. the ratio of growth suddenly (within a 



