76 OUTLINES OF CHORDATE DEVELOPMENT 



become differentiated and from it radiations begin to pass 

 downward into the cytoplasm. This area extends rapidly 

 and the radiations pass around the nucleus and up toward the 

 animal pole of the egg as well as centrally. Finally they ex- 

 tend even through the nuclear membrane and the nuclear 

 meshwork takes on the same radiational arrangement which 

 thus involves practically the entire animal pole. The nuclear 

 membrane dissolves as the radiations become complete and 

 the nucleoli are aggregated or dissolved, and just at this time 

 the egg follicle is ruptured in some way, and the egg escapes 

 freely into the body cavity surrounded only by its chorionic 

 and vitelline membranes (Fig. 23, C). 



Certain areas of the peritoneum are covered with cilia 

 which beat in the direction of the oviducal ostia. These too 

 are abundantly ciliated, and as a result of the ciliary action 

 in both regions the eggs are soon carried to and into the upper 

 ends of the oviducts. But by this time the first polar spindle 

 is already formed. 



As the egg leaves the ovary the small group of chromatin 

 nucleoli becomes surrounded by a small spherical mass of 

 fibrillar plasma; the nucleoli become more or less fused and 

 vacuolated, and then give rise to the group of twelve small 

 rod-like chromosomes which soon become rings or crosses (Fig. 

 26, F). The fibrillar plasma draws out into the elongated 

 achromatic spindle, at first placed tangentially (Fig. 26, G), but 

 soon rotating and coming to the surface of the cell in the radial 

 position (Fig. 26, H). The spindle is quite blunt and no asters, 

 centrosjgheres, or centrosomes have been seen. The chro- 

 mosomes diverge as the egg is entering the oviduct, in the 

 upper part of which the very small first polar body is cut .off. 

 (Fig. 26, /). 



The second polar spindle forms immediately, and by the 

 time the egg reaches the lower end of the oviduct the second 

 polar division has progressed as far as the mesophase or meta- 

 phase. In this condition the division is suspended, and pro- 

 ceeds considerably later and as a rule only after entrance of 

 the spermatozoon. 



