GERM CELLS IN PEDICELLINA AMERICANA 31 



mosotnes and stains rather lightly in comparison with that of 

 the true oocytes. 



V. Fertilization and Cleavage. 



The ripe spermatozoa cast out into the water make their 

 way into the atrium of a female polyp, thence into the ovary, 

 where the eggs are fertilized. Spermatozoa are often found a 

 considerable distance within eggs where the chromatin is still 

 long before concentration (Fig. 80, PI. II, sp.). In most cases, 

 however, fertilization occurs when the first polar spindle is 

 about to be formed. With the latter in the metaphase, the 

 fertilized egg makes its way through the oviduct, into the 

 atrium, where the brood pouch is formed. Here the remainder 

 of embryonic development is passed through until the free 

 swimming larva is attained. 



In the act of fertilization, the head alone enters the egg (Fig. 

 80). The head, however, as was shown in the preceding sec- 

 tion, is a composite structure, containing at its posterior end 

 the middle piece with which it had fused. From the compar- 

 ison of many instances, it appears that the sperm may enter the 

 egg at any point. Once within, it makes its way through a 

 mass of yolk spheres, leaving behind it a track of pure cyto- 

 plasmic substance. It is in this area that the sperm-aster with 

 its centrosome and centriole lie. The head now concentrates 

 to an even smaller size, losing at the same time, its lanceolate 

 form and giving very much the appearance of one of the yolk 

 spheres among which it lies. The astral system is no longer to 

 be observed. 



In the meantime the maturation processes are being passed 

 through. With these at an end, the sperm head swells up and 

 soon becomes of equal size with the female pronucleus from 

 which it can at this time be distinguished only by the proximity 

 of the latter to the newly formed polar bodies (Fig. 106). 

 Within both, the chromatin is often in the form of a finely di- 

 vided reticulum supported and connected throughout by a con- 

 tinuous linin system. As the nuclei approach each other they 

 grow larger and the chromatin reticulum concentrates into 



