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PHYLUM ARTHROPODA. 



Arthropoda. The spherical ovum is surrounded by a cuticular vitelline 

 membrane, and contains a considerable quantity of yolk. After ferti- 

 lisation the segmentation nucleus divides in the usual way into two, 

 four, eight, and so on, but this nuclear division is not followed by 

 division of the plasma. Eventually the nuclei, each surrounded by a 

 small amount of protoplasm, approach the surface of the egg and 

 arrange themselves regularly round it. The peripheral protoplasm then 

 segments round these nuclei, and thus we have a central core of un- 

 segmented yolk enveloped by a peripheral sphere of rapidly dividing 



ch 



FIG. 147. Section through the egg of Astacus after the com- 

 pletion of segmentation. After Reichenbach. 



//., Stalk of the egg ; ch., chorion envelope ; <5/., peripheral blastoderm 

 within which are the yolk pyramids (dark). 



cells. In the central yolk, free nuclei are frequently found ; these are 

 the so-called yolk nuclei. Such a type of segmentation is called peri- 

 pheral or centrolecithal, and is very characteristic of Arthropod eggs. 



Over a particular region of the segmented egg, known as the " ventral 

 plate," the cells begin to thicken ; at this region an invagination occurs, 

 which represents the gastrula. At the anterior lip of the blastopore the 

 mesoderm appears, being many-celled from the first. Soon the blasto- 

 pore closes ; the cavity of the gastrula thus becomes a closed sac the 

 future mid-gut. The cells of this archenteron take up the core of yolk 



