AMPHIOXUS 

 A 



AMPHIBIAN 

 B 



BIRD 



MAMMAL 



Fig 84.— Sample stages of cleavage and gastrulation in eggs of chordates. 

 A. Amphioxus, cleavage holoblastic, little yolk; egg diameter 0.1 mm (After 

 Hatschek.) B, Frog, modified holoblasUc cleavage, much yolk; diameter 2 mm. 

 fvSous sources.) C, Bird, meroblastic discoidal cleavage in small Was to disc on 

 large yolk mass; diameter 30 mm. (After Blount; and Patten, Embryo ogy of the 

 Chfck The Blakiston Co.) D, Mammal, cleavage holoblastic, practically no yolk 

 an outer and inner cell mass formed in blastula, 6; gastrula formed by migration of 

 entoderm cells from inner cell mass, 7; egg surrounded during early cleavage by 

 zona pellucida (from Graafian follicle of ovary) which later disappears; diameter 

 2 mm (After Gregory; and Patten, Embryology of the Pig The Blakiston Co.) 

 (From Storer: General Zoology, ed. 2, 1951, McGraw-Hill Book Co., Inc.) 



