FEMALE REPRODUCTIVE CELLS 



467 



435, A, shows such a new spindle with the chromosomes at metaphase. 

 The bi-lobed chromosomes are again divided longitudinally into daughter 

 chromosomes, which are also bi-lobed. Figure 435, B, shows a telophase 

 of such a figure which soon throws off a second polar body exactly as the 

 first figure did its first polar body (Fig. 435, C). In this case the first 

 polar body is apparently making an abortive attempt to divide. Shortly 

 after this the egg chromosomes expand into vesicles which fuse to form 

 the female pronucleus. 



Figure 435, D, shows the two polar bodies completed and the egg 



'chromatin forming a nuclear membrane. In this case the first polar 



body has made no attempt to divide. The egg aster still persists, and 



the whole figure appears as an almost perfect resting nucleus with a 



centrosome. 



A remarkable point in the whole growth period of these eggs is the 

 lack of well-developed nurse cells. The follicle cells show only a very 



FIG. 435. Asterias Forbesii. Four stages to show the second reduction division and the 

 formation of the female pronucleus (D). (Drawn by H. E. JORDAN.) 



thin layer, complete, but so delicate that they do not seem to have the 

 power of preparing the great quantity of yolk that the ova acquire. A 

 possible explanation of this condition is the fact that the ovaries lie 

 bathed in the general blood supply, and that this has such free access 

 to the ova that they can absorb sufficient food material directly from its 

 body. This presupposes that the blood is very rich in a food supply 

 which needs but little elaboration to become yolk. 



When the egg has matured, and is brought into the proximity of 

 sperm, the spermatozoa are influenced by its presence to direct them- 

 selves toward it, and to make energetic swimming efforts to reach it. 

 Upon reaching it, the first spermatozoon forces its way through the thick 

 zona radiata at the micropyle, a small opening which is there for that 

 purpose. As it draws near the ovum the latter responds to its first con- 

 tact by a lifting of the cytoplasm at that point in the form of a cone, into 

 which the sperm passes. It continues to move into the cytoplasm of 

 the ovum, but leaves its tail behind at the surface. This surface now 

 becomes covered with a delicate membrane, the mtelline membrane, 



