50 



The Development of the Vertebrate Embryo 



THE FORMATION OF GAMETES: THE EGG 

 The egg has three functions to perform: 



1. To supply a nucleus containing half the chromosomal comple- 

 ment of the future embryo. 



2. To supply almost all the cytoplasm upon union with the sperm 



3. To supply food reserves that will enable the embryo to develop 

 to a stage where it can begin to feed upon exogenous materials. 



Because it must do all these things, the animal egg is a cell of giant size. 

 Consider for example, the size of a hen's egg. Even that of the mammal, 

 .U^.25 mm, IS considerably larger than ordinary body cells 



The egg is covered by a protective envelope: in birds, by an in- 

 organic shell in amphibians, by a ielly coat made up of polysaccharide 

 and protein of high molecular weight. In mammals, a single layer of very 

 small protective cells cover the egg surface. The egg proper is bounded 

 by one or more membranes. A single haploid nucleus resides within to- 

 gether with cytoplasmic constituents whose fine structure, revealed bv the 

 electron microscope, does not seem to be diflFerent in kind from the'con- 

 tituents of other cell types. Finally, there is yolk, the food supplv for 

 .1T^^T I ''^^I'^'^^^^^ous mixture of fat droplets, granules! and 

 small bodies, bounded by membranes. Some of these are highlv pigmented 

 and, because the yolk is organized material, can be seen to occupv definite 

 regions of the egg and to be parceled out amongst the daughter cells in a 

 aelimte manner. 



r„.»?r'' T ^'"^'^ differences among eggs of the animal species with 



e kt V IvMri XT ^"^ "r"'^' *^ ''Sgs of marine animals contain 



relat,vely htt le yolk because the embryo begins active feeding at a very 



us Iv ZT T """ *T '^ '" "^""^ ^gg^ -^ *^'"buted homogenZ 

 of bL "f .^""^^"'^"'^d i" - Particular region. In contrast, the eggs 



Unit IT"V°T"' T°™'^ °f y""^- ^°^ «^"P'<=' 'h-' ''en's egg 



cell f% ^^' ^'» ~nt^'n large amounts of yolk, which is situated at the 

 these forl^'th^'' u surrounded by a thin outside layer of cytoplasm. In 

 before eme '" " '^'"' considerable developmental changes 



before emergmg as an actively feeding organism; hence the large supply 



mamma fanTb */ *T "' *^ -°'"*»"-y -ale are the mammafs'I 

 and reci ve T ™'°P' '" " ^^"^'^ '' ' ?""-'« '"-de its mother 



A lar^voir ' "'"'\'" '■' " ^"^ '"^' ^"""'e' °f h« ■■"'crnal organs, 

 ^eribfetr 'T™ " '^'"'''"' unnecessary, and mammalian eggs re- 

 semble the primitive marine eggs in this respect. 



