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AN INTRODUCTION TO ZOOLOGY 



no doubt secreted by the cement glands. This is brought about 

 by the turning of the animal first on one side and then on the 

 other a number of times. From one hundred to over six hundred 

 greenish eggs are laid by a single female, depending upon the 

 size and age of the animal. After the eggs are laid the crayfish 

 rights herself, the apron of glair breaks down, and the abdomen 

 is extended (Fig. in). 



FIG. in. Female aerating eggs by raising and straightening abdomen and 

 waving swimmerets back and forth. (From Andrews in Am. Nat.} 



FERTILIZATION. The method of fertilization has never been 

 discovered. It is supposed that as the eggs are laid they pass 

 over the opening of the seminal receptacle, and are then pene- 

 trated by the spermatozoa which were placed there by the male 

 the preceding autumn (160). 



Embryology. While the eggs are developing they are pro- 

 tected by the abdomen of the female, and are aerated and kept 

 free from dirt by the waving of the swimmerets back and forth 

 (Fig. in). The embryology of Cambarus has never been inves- 

 tigated, but it probably resembles closely the development of the 

 common European crayfish, Astacus flumatilis. 



The fertilized egg of Astacus consists of a large number of 

 yolk spheres embedded in cytoplasm, and contains a nucleus 

 composed of the egg nucleus and the spermatozoon combined. 

 This nucleus proceeds to form two daughter nuclei. The egg 

 does not become divided into two parts by cell walls, but the 

 daughter nuclei separate from one another and divide again 



