CRAYFISH 109 



down the tubules and eventually leave the testis through the 

 right or left vas deferens. 



Female Organs of Reproduction. The eggs are developed in 

 a bilobed ovary which is situated in the same general region in the 

 female as is the testis in the male. Leading from the ovary are two 

 tubes (oviducts), each of which opens to the exterior through the 

 coxopodite of the third walking leg. The ovary of the Crayfish 

 contains a central cavity connected with the oviducts. The eggs 

 are formed from the definitive germ cells located in the wall of 

 the ovary. When ripe they break loose from the wall into the 

 central cavity from which they are taken to the exterior by the 

 oviducts. 



Development of the Crayfish. The breeding season of the 

 Crayfish is in the fall. At this time the two sexes pair, and sperm 

 cells are transferred from the male to the female by means of the 

 specialized abdominal appendages. The sperm cells remain during 

 the winter in a small cavity, the seminal receptacle, situated 

 on the ventral surface of the body of the female, between the fourth 

 and fifth walking legs. The following spring, the mature eggs 

 developed in the ovary are laid, after being fertilized by the sperm 

 from the male which were previously stored in the seminal recep- 

 tacle. The fertilized eggs are attached by a sticky secretion to 

 bristles present on the abdominal appendages, with the result 

 that almost the entire ventral surface of the abdomen of the female 

 may be covered with the developing eggs, each enclosed in a 

 special protective capsule. 



When the egg has been fertilized, cell division begins. However, 

 the entire fertilized egg, or zygote, of the Crayfish does not divide 

 at first, but only the fertilization nucleus, or synkaryon. The 

 latter divides to form a number of nuclei which lie in the center of 

 the developing zygote. These nuclei later migrate to the surface 

 of the zygote, and then the cytoplasm divides radially into as 

 many parts as there are nuclei. Division at right angles then 

 occurs in such a manner as to cut off a small peripheral portion of 

 the cytoplasm, surrounding each nucleus, from each of the radial 

 parts. Thus, at this stage, the embryo consists of an outer, or 

 surface, layer of cells which contain the nuclei and enclose an inner 

 mass of yolk material. The latter does not play any direct part 

 in the further development of the embryo, but supplies the nec- 

 essary nourishment. 



