CRAYFISH. 



279 



her back, and deposits the seminal fluid on the ventral 

 surface of the abdomen. The fluid flows down the canal 

 formed by his first abdominal appendages, and these seem 

 to be kept clear by the movements of the next pair, which 

 are also modified. On the abdomen of the female the 

 agglutinated spermatozoa doubtless remain until the eggs 

 are laid, when fertilisation in the strict sense is achieved* 



FIG. 130. Section through the egg of Astacits after the com- 

 pletion of segmentation. After Reichenbach. 



st., Stalk of the egg ; ch., chorion envelope ; II., peripheral blastoderm 

 within which are the yolk pyramids (dark). 



The Development has been very fully worked out, and is of interest 

 in being direct, without the metamorphosis so common among the 

 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 



