CHAPTER 111 



THE PROCESS OF SEGMENTATION 



The effect of yolk on segmentation; the unsegmented 

 blastodisc; the sequence and orientation of the 

 cleavage division in birds. 



The Effect of Yolk on Segmentation. — Immediately after 

 its fertilization the ovum enters upon a series of mitotic divisions 

 which occur in close succession. This series of divisions 

 constitutes the process of segmentation or cleavage. In birds 

 segmentation takes place before the egg is laid, during the time 

 it is traversing the oviduct. 



A mitotic division, whether it be a cleavage division of the 

 ovum or the division of some other cell, is carried out by the 

 active protoplasm of the cell. The food material stored in an 

 egg cell as deutoplasm is non-living and inert. The deutoplasm 

 has no part in mitosis except as its mass mechanically influences 

 the activities of the protoplasm of the cell. It is obvious that 

 any considerable amount of yolk will retard the division, or 

 prevent the complete division, of the fertilized ovum. The 

 amount and distribution of the yolk will therefore determine 

 the type of segmentation. 



Figure 4 shows diagrammatically the manner in which the 

 first cleavage division is carried out in three types of eggs 

 having different relative amounts and different distributions of 

 yolk and protoplasm. In the egg of Amphioxus the yolk is 

 relatively meager in amount and fairly uniformly distributed 

 throughout the cell. An ovum with such a yolk distribution is 

 termed isolecithal (homolecithal) . An isolecithal egg under- 

 goes a type of cleavage which is essentially an unmodified 

 mitosis. The yolk is not sufficient in amount, nor sufficiently 

 localized to alter the usual mode of cell division. 



In Amphibia the ovum contains a considerable amount 

 of yolk and the accumulation of the yolk at one pole has crowded 

 the nucleus and active cytoplasm of the ovum toward the 

 opposite pole. An egg in which the yolk is thus concentrated 



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