180 



GENERAL EMBRYOLOGY 



it pauses to await the sperm entrance. In Sycandra, Lepas, 

 Amphioxus (Fig. 90), and many Amphibia and Mammalia the 

 first polar division is completed and the second polar spindle 

 formed before the pause. And finally in some Ccelenterata 

 and most Echinodermata and Ascidians, the maturation of 

 the egg is entirely completed before the entrance of the sperma- 

 tozoon. In such cases as these, contact with sea water seems 



FIG. 93. Diagrams of the two most frequent relations between the events of 

 maturation and fertilization. From Wilson, "Cell." I. Polar bodies formed 

 after the entrance of the spermatozoon (Annulates, Molluscs, Turbellaria) . //. 

 Polar bodies formed before the entrance of the spermatozoon (Echinoderms). A. 

 Sperm pronucleus and centrosome at d\ first polar body forming at 9 . B. 

 Polar bodies formed; approach of the pronuclei. C. Union of the pronuclei. 

 D. Approach of the pronuclei. E. Union of pronuclei. F. Cleavage nucleus. 



to furnish the stimulus to complete the maturation process, 

 which may be begun before the eggs are produced. 



We may distinguish two general types of behavior on the 

 part of the germ nuclei according to whether maturation has or 

 has not been completed at the entrance of the sperm (Fig. 93). 

 We shall consider first, and at greater length, those cases in 

 which maturation is not yet completed, for this would seem 

 the more usual course of events. Otherwise maturation occurs 

 precociously, apparently before the necessity for it has arisen. 



