I I 

 Cleavage and Differ entiatio?i 



!/ HE ESSENTIAL PROBLEM OF ANIMAL DEVELOPMENT LIES 



in the question: How does the egg, a single cell, become an 

 adult organism? If one take the transparent eggs of a 

 common marine fish like the mackerel which float like 

 bubbles on the surface of the sea, one can easily follow their 

 development under low power of the microscope. Before 

 fertilization a thin film of cytoplasm beneath the membrane 

 encloses a core of yolk and oil. With fertilization this 

 cytoplasm flows to one egg-pole to collect there as a disc.^ 

 This disc is next crossed by one furrow and then by another 

 at right angles to it; so by this cleavage two and later four 

 cells arise. Cleavage progresses until many cells form, 

 whilst some of the yolk beneath the cytoplasmic disc is 

 transformed into cytoplasm. Soon one discerns an opaque 

 line running the length of the disc. Here there are more 

 cells than elsewhere, hence the opacity; from some of these 

 will come the future embryo. Under the microscopic even 

 one who has very little knowledge of the development of 

 eggs can follow the origin and formation of the nerve tube 

 out of which the brain and spinal cord emerge. One looks, 

 as through an open window, at the very mystery of life, 

 wondering at the heartbeat, first uncertain and then in 

 definite, sure rhythm; the bright red blood moving in jerks 

 with each beat of the heart; the first spasmodic muscular 

 twitch; the appearance of the purple-black eye-pigment; 

 the definite fish form; the color of the skin which will give 



^ Cy. Ransom, /cS'57. 



286 



