STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 53 



alone undergoes cleavage. The relatively large size of these cg^s is 

 therefore attributable to the quantity of yolk that they carry. To 

 this group belong the eggs of birds, reptiles, fish and amphibians, 

 and also the oviparous mammals, the monotremes. The eggs of all 



Fig. 37 



Armadillo oocyte (Dasyptt* novemcinctus), showing segregation 

 of yolk. (Drawn from illustration by Newman, 1912.) 



other mammals are typically miolecithal, the yolk being much 

 scantier and to varying degrees mingled with the cytoplasm; the 

 whole vitellus takes part in cleavage. Variations in the size of 

 miolecithal mammalian eggs are evidently due in no small measure 

 to variations in the mass of active cytoplasm, for larger eggs in this 

 series have larger nuclei. In the egg of the native cat Dasyurus (Fig. 

 io) and the armadillo Dasypus (Fig. 37), much of the yolk in the 

 oocyte and ootid is gathered at one pole and forms a separate body 

 during early cleavage. Suggestions of polarity in the arrangement 

 of the yolk components are seen also in other mammalian eggs, 

 such as those of the guinea-pig (Fig. 38), but here the yolk is disposed 

 as globules or droplets. In the eggs of some bats (Fig. 39), the cat 

 (Figs. 19, 20, 40 to 45), the ferret, the dog (Fig. 46), the fox and the 

 pig, very numerous globules are distributed more or less uniformly 

 throughout the vitellus. The eggs of man, monkey, the horse, the 

 cow, the sheep, the rabbit and the murine rodents mostly have a 

 granular yolk with a pattern of distribution characteristic for each 

 species. In certain inbred strains of mice, it has been shown that the 

 pattern is recognizably different with each strain (Braden, 1959)- 



