Embryology 283 



cleavage is confined to a small germinal disc at the animal pole. So 

 dense is the yolk in these eggs that the cleavage planes do not occur at 

 the vegetal pole. 



NEURAL 

 TUBE 



EPIMERE 



MESOMERE 



SPtANCHNIC 

 MESODERM 



Fig. 85. — Diagrams showing the origin of mesoderm and notochord and the 

 formation of the neural tube. A, Cross section of embryo showing the beginning 

 of the outpocketing of the mesoderm and the dorsal thickening of the neural 

 plate; B, cross section of later stage in which notochord is separated and the 

 mesoderm is differentiated into epimere, mesomere, and hypomere; the neural tube 

 is formed; C, cross section of later stage in which the epimere has separated 

 from the mesomere, the mesenteries are forming; D, sagittal section to show 

 relationships of structures; is in approximately the same stage of development as 

 B. Ectoderm indicated by cross lines, entoderm by coarse stipple, mesoderm by 

 fine stipple, and notochord by solid black. 



On the basis of these differences in amount of yolk, eggs and 

 their cleavage patterns are classed as follows : 



1. Eggs with a small amount of yolk which is evenly distributed: isoleci- 

 thal eggs. Cleavage involves the entire egg and results in blastomeres of ap- 

 proximately equal size: holoblastic and equal cleavage. Example: Amphioxus. 



2. Eggs with a moderate amount of yolk which is concentrated at the vegetal 

 pole: moderately telolecithal eggs. Cleavage involves the entire egg, but the 

 blastomeres formed at the vegetal pole are larger and fewer than those at the 

 animal pole: holoblastic and unequal cleavage. Example: frog. 



