140 COMPARATIVE ANATOMY 



abdominal segments of the female, where they may be seen 

 adhering as a white mass. The ova are formed from the 

 germinal cells lining the walls of the saccular ovary. Certain 

 of these cells increase in size and become ova; they are 

 surrounded by a follicle and project into the cavity of the 

 ovary, into which, when ripe, they fall by the bursting of the 

 follicle, and pass to the exterior through the oviducts. During 

 their passage through the oviducts the ova are invested by a 

 viscid transparent substance, and on passing to the exterior 

 are attached by short stalks of this substance to the abdominal 

 limbs. A female crayfish may often be seen with a mass of 

 eggs fastened beneath the abdomen, and is then commonly 

 said to be "in berry." The manner in which the ovum is 

 fertilised by the spermatozoon is not known, but segmentation 

 sets in soon after the eggs are extruded, and all the principal 

 stages of development are passed through whilst the eggs are 

 attached to the mother. 



The ovum of the crayfish is abundantly supplied with food- 

 yolk, in which the nucleus occupies a central position. The 

 nucleus divides repeatedly, and the products of division pass 

 to the periphery of the ovum, where they form a layer of 

 nuclei embedded in a continuous sheet of protoplasm not 

 yet segmented into separate cells. The protoplasm and yolk 

 are then divided into as many elongated cells as there are 

 nuclei. Each cell is called a yolk-pyramid, reaches from the 

 periphery nearly to the centre of the ovum, and is made up 

 of an external protoplasmic part containing the nucleus, and a 

 large internal part consisting of little else than yolk (fig. 33, C). 

 In the next stage the protoplasm surrounding the peripheral 

 nuclei is separated to form a single layer of cells or blastoderm 

 investing a central mass of yolk formed by the fusion of the 

 inner ends of the yolk-pyramids (fig. 33, D). An oval thicken- 

 ing, known as the primitive streak, is now formed on what will 

 be the ventral side of the blastoderm, and immediately after- 

 wards five other thickenings are formed near it viz. a pair 

 of rounded areae at its anterior end, the optic lobes ; a pair of 

 lateral thickenings called the thoraco- abdominal plates, and 

 posteriorly a median thickening called the endoderm plate, 

 which is continuous anteriorly with the primitive groove. In 

 the next stage the endoderm plate is invaginated, forming a 

 small pocket projecting into the yolk and opening to the 



