4ia 



THE MAINTENANCE OF SPECIES 



amphioxus and of mammals contain but a small amount of equally 

 distributed nutritive material, while a third type of distribution 

 occurs in some insect eggs where the yolk is concentrated in the center 

 of the ovum. 



Blastulation 



In isolecithal eggs, in which the yolk is distributed throughout the 

 egg, the cells produced by successive divisions are all of approxi- 

 mately the same size, and cleavage progresses with regularity until 



, , /Polair body, 



polar Dod^ ^ -^ 



" :rivi tell ins 

 space 



fertili3atibn: 



arcbenteron -^ 

 ectoderm^ blcrstocoel 



ventrcU 

 lip 



blastopore ^' 



ET .. . ^ 



Cleavage in Amphioxus. Note fertilization membrane (I) and decrease in cell 

 size as blastulation occurs (II-IV). Gastrulation (V, \T) follows with a reduction 

 of blastocoel and formation of gut {archenleron). (After Conklin.) 



the embryo is a mass of increasingly smaller undifferentiated cells. 

 A central cavity is produced as soon as the scanty yolk is used 

 up to furnish fuel for cell division. As a result the entire mass re- 

 sembles a rubber ball with the surface representing the layer of out- 

 side cells and the cavity inside of the liall forming the hlastocoel. 

 This stage is called a hlastula, and the process whereby it is formed 

 is known as blastulation. 



Gastrulation 



As mitosis continues after blastulation, the cells on the side con- 

 taining the yolk gradually become larger and eventually are pushed 

 inward much as one would push in the side of a hollow rubber ball 

 with the finger. The new cavity thus formed represents the primitive 

 gut, or archenleron, and the embryo is now spoken of as a gastrula. 



