CLEAVAGE AND DIFFERENTIATION 



the markings that make the adult mackerel one of the most 

 beautiful creatures of the sea. 



There are other eggs, those of ascidians, which like the 

 mackerel belong to the highest group in the animal king- 

 dom, that hatch as swimming larvae in eight hours after 

 fertilization instead of seventy-two hours, the time the 

 mackerel Qgg in a warm laboratory requires. From egg to 

 complex animal in eight hours 1 The problem of the devel- 

 opment from a single cell to a complex animal is thus a 

 fascinating one. 



In the following exposition we shall see again that exact 

 description of the process which we aim to explain plays 

 a chief and significant role. Whilst here as with many 

 other problems in biology we must realize that a description 

 does not explain the process described, nevertheless descrip- 

 tion is the prerequisite of a successful attack on the 

 problem of development, a problem which doubtless more 

 than any other in biology is complicated by the interplay 

 of simultaneously occurring reactions and by rapidly suc- 

 ceeding events. To determine cause and effect in this 

 manifold process of differentiation is a task whose difficulty 

 is so manifest that we must before all else seek to follow 

 the process as exactly as possible in closely set stages. 

 We may then be able to relate smallest changes in form and 

 in expression to general phenomena of development, that 

 is, to the differentiation which accompanies the cleavage- 

 process. Such a task is undertaken in this chapter. 



Following the classification of Karl Ernst von Baer, one 

 can separate the development of fertilized or parthenogenetic 

 animal eggs into four periods: cleavage, formation of the 

 germ-layers, development of the organs and histological 

 differentiation. This classification represents a scheme 

 which holds more nearly true for the higher animals than 

 for the lower and should not be taken as meaning that the 

 development of any egg is sharply divisible Into fixed 



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