240 GENERAL ZOOLOGY 



spermatozoa. Fertilizin seems to sensitize the sperm and as a 

 result of this the sperm is capable of inducing reactions in the 

 egg, leading to engulfment of the sperm when the latter touches 

 the egg. Penetration of the egg by the spermatozoon is not a 

 mechanical boring process. 



Entrance of the Sperm. — The ootid is the final stage of oogene- 

 sis, but in some cases the formation of the polocytes, which marks 

 the completion of the maturation divisions, is not accomplished 

 until after the egg (oocyte) has left the ovary. There is no 

 general rule regarding the stage of development reached by the 

 egg before it is laid or fertilized. In some species the egg is ready 

 for impregnation or insemination by the sperm at the primary 



A B C 



Fig. 147.- — Entrance of the spermatozoon into the egg of the starfish, Asterias. 

 (After Fol.) A, a single spermatozoon (s) has penetrated the jelly (j) and 

 become attached to the fertilization (entrance) cone (c). B and C, succeeding 

 stages in the engulfing of the spermatozoon, m, fertilization membrane. 



oocyte stage, in which case the sperm enters before the polocytes 

 are formed. In fact, unless the sperm does enter such an egg, 

 no maturation divisions take place. In other cases, the sperm 

 enters at the secondary oocyte stage, and in still others in the 

 ootid stage. 



Egg of Nereis. — The reactions of the egg that follow or result 

 from the contact of the sperm with the egg are well illustrated 

 in the case of the marine annelid, Nereis. When the egg of 

 Nereis is deposited in the water, it is in the primary oocyte stage, 

 neither polar body having been formed. The nucleus occupies 

 a central position and the cytoplasm contains numerous yolk and 

 oil globules. The egg is bounded by a thin vitelline membrane 

 which is its only covering when it is laid. The spermatozoon 

 consists of a flattened head, middle piece, and tail. The apex 

 of the head is provided with a short spike, called the perfora- 

 torium, by means of which the sperm attaches itself to the egg 



