28 Fertilization and Early Development of Pigeon's Egg 



If the cause assigned by Eiickert for the occurrence of physiological 

 polyspermy be correct, namely, the absence of protection against it on 

 account of the thinness of the egg covering in these internally fertilized 

 eggs, it might well be expected to occur in the bird's egg also. Balfour, 

 85, stated in regard to the chick, that " In the bed of white yolk nuclei 

 are present which are of the same character and have the same general 

 fate as in Elasmobranchs. They are generally more numerous in the 

 neighborhood of the thickened periphery of the blastoderm than else- 

 where." 



Among the amphibia, polyspermy has been found in the urodela. Thus 

 Jordan, 93, found it to be universal in the newt. He states that " there 

 is every reason for regarding such physiological polyspermy in the newt 

 as a natural, normal and in fact usual occurrence." The extra nuclei 

 degenerated shortly after the fusion of the pronuclei. Pick, 92-93, foimd 

 polyspermy occurring in axolotl, but inconstant. Braus, 95, in triton 

 found the sperm nuclei dividing amitotically from the start, a fact which 

 Riickert correlates with the entrance of the spermatozoa through the 

 yolk, since in elasmobranchs the sperms enter through the germinal disc 

 and change from the mitotic to the amitotic method of division after they ■ 

 have migrated into the yolk. The anura, on the other hand, are mono- 

 spermic according to the evidence of Hertwig, Born, 86, Eoux, 81, and 

 King, 01, although opposing observations were recorded by Kupffer, 82, 

 in the case of Bufo. 



In the elasmobranchs the yolk nuclei have been the subject for much 

 controversy and speculation. Balfour, 74, recognized the existence of 

 such nuclei in the late cleavage stages of the selachian blastoderm. He 

 surmised that they arose spontaneously in the yolk. Schultze, 77, dis- 

 agreed with such an assumption as to their origin and took it for granted 

 that they arose from the cleavage nuclei. Eiickert, 85, traced these free 

 nuclei as far back in development as the eight-cell stage, which was the 

 earliest stage he found. He argued that they arose from an equatorial 

 cleavage of the nuclei of the four-cell stage, and that their position in 

 the yolk indicated that they were the homologues of the nuclei of the 

 vegetative pole in the frog's egg. Their peripheral position was taken 

 as strongly favoring such an homology. Eiickert termed them "mero- 

 cytes," indicating thereby that they were parts of cells, namely, the 

 nucleus with some surrounding protoplasm, which after division migrated 

 away from the cellular region into the yolk. Kastschenko, 88, found 

 these merocytes in the stage of the formation of the first furrow. He 

 proposed a theory that the first cleavage nucleus gave rise to a multi- | 

 nucleate Plasmodium before the first division of the egg. Eiickert, 90-92, 



