V THE DEVELOPMENT OP^ THE FROG 87 



In Rana fiisca the second polar body is given off after 

 tlie egg has been laid, and within half an hour after fertiliza- 

 tion ; no resting stage of the nucleus intervenes between the 

 two maturation divisions, and in the second division, as in 

 the first, half the chromatin material of the egg is extruded. 

 The two polar bodies in Rana fusca are of about equal size, 

 and are either attached to the animal pole or float in the 

 fluid which accumulates between the egg and the vitelline 

 membrane. If the jelly is removed from a freshly laid egg, 

 and the light spot, or fovea, at the animal pole observed 

 with a hand lens, the extrusion of the second polar body 

 may be witnessed. , 



When the maturation divisions are completed, the amount 

 of chromatin in the egg has become much reduced and the 

 number of chromosomes diminished by one half. Whether 

 the chromosomes in the matured egg are double, so that the 

 reduction in number is only apparent, or whether there is an 

 actual loss of half of the chromosomes is a question still 

 under discussion. 



Fertilization. — The act of fertilization consists in the union 

 of two cells, the o%>um from the female and the spermatazooji 

 from the male. As bearers of hereditary qualities these 

 two cells are equal, but in size and form they are as dissimilar 

 as they can well be. The sperm cell is a minute, elon- 

 gated body consisting of a narrow, pointed head formed 

 mainly of the nucleus, a short middle piece just behind the 

 head, and a long, very slender iaiL As before described, 

 the seminal fluid of the frog is shed over the egg masses as 

 they are extruded into the water from the body of the female. 

 The spermatozoa which swarm in the seminal fluid are active, 

 and swim about by the movements of the tail, working their 

 way through the jelly of the egg mass until one comes in 

 contact with an egg. Then the spermatozoon slowly pene- 



