32 Fertilization and Early DtTelopment of Pigeon's Egg 



nuclei then are surrounded by entirely similar areas. They are at this 

 stage limited in their movements by the cell boundaries, and so con- 

 fined to very small areas. Consequently they become surrounded by 

 cytoplasm, which is the product of their own activity in altering the con- 

 stituents of the yolk. This material is highly plastic and responds to the 

 forces operating in mitosis so as to produce regular figures. 



Compare with the limited movements of these nuclei the wide migra- 

 tions of the nuclei resulting from the first cleavage, and we see that the 

 latter have no chance to become surrounded by an altered material, since 

 all products of nuclear activity must rapidly become diffused into the 

 surrounding cytoplasm. The reason for the different appearance in 

 mitosis is seen when we compare the alveolar structure of the unaltered 

 egg cytoplasm with that surrounding the later cleavage nuclei. The 

 original egg cytoplasm is coarser, i. e., the alveoli are larger, and takes 

 the cytoplasmic stains much less deeply. This is not very apparent m 

 the drawings. In the .maturation stages the absence of a metabolic 

 phase of the nucleus for so long a period makes the surroundings of the 

 nucleus least favorable of all apparently for the production of typically 

 regular figures. It has been noted by some observers that the second 

 maturation division differs from, the first in the poorer development of 

 asters, a phenomenon which might be due to the altered character of the 

 surrounding cytoplasm. It would seem in the case of the pigeon's egg 

 that the hyaline zone surrounding the first polar spindle (Eig. 6a) is 

 not reformed so conspicuously at the second division, there being only 

 scattered vacuoles present at this period in the region surrounding the 

 nucleus. The suggestion that the deutoplasmic granules surrounding 

 the nucleus in these early stages inhibit the formation of achromatic 

 structures is perhaps an incomplete explanation, since the nature of the 

 C3rtoplasmic groundwork may be a more fundamental cause. 



Conclusions. 



1. As a result of the monogamous habit of pigeons, ovulation is nor- 

 mally held in abeyance till aroused by the stimulus received from the 

 male. The passivity of the female is compensated by the highly devel- 

 oped and complex instincts of the male bird. The determination of the 

 time of fertilization and egg-laying must date from the time of mating. 

 The second egg of a pair is set free from the ovary and enters the 

 oviduct within a few hours 'after the first is laid. The egg is impreg- 

 nated before entering the oviduct. 



2. Polyspermy is normal. The most favorable region for entrance of 

 sperms is the " fovea," in a zone surrounding the egg nucleus. Never 



