32 HISTOLOGY 



The difficulty of counting chromosomes is apparent from the varying 

 numbers which have been reported in the mouse After reduction the 

 number has been placed at 8, 12, 16, 18 and 20 by different observers. 



The polar bodies in the mouse are relatively large. In the upper part 

 of Fig. 24, A, a polar body is about to be formed, and it is completely cut 

 off from the oocyte in Fig. 24, C. In D and G, two polar bodies are shown. 



FERTILIZATION. 



In the mouse, from six to ten hours after coitus, spermatozoa have made 

 their way to the distal end of the uterine tube, where fertilization takes 

 place. According to Long and Mark, the maturation of ova usually occurs 

 at some time during the period from "13! to 28^ hours " after the mouse has 

 given birth to a Utter; and during the process of their maturation, the 

 oocytes are discharged from the ovary and enter the distal end of the tube. 

 Here, if fertilization takes place, a single spermatozoon penetrates the zona 

 pellucida. In a section obtained by Sobotta, the entrance of the sperma- 

 tozoon has been partially accomplished (Fig. 24, B). Its tail lies outside 

 of the zona, and appears to have become thickened. In another specimen 

 Sobotta found the head, middle piece and a part of the tail within the 

 cytoplasm of the oocyte. The tail had broken as it crossed the zona, and 

 the portion remaining outside had drawn together and was disintegrating. 

 In some animals it is said that the entire spermatozoon enters the ovum, 

 but in others only the head and middle piece. In any case the tail appears 

 to be a propelling apparatus which becomes functionless after the head 

 and middle piece have passed through the zona. It has entirely disap- 

 peared in the stage shown in Fig. 24, A, in which the head of the spermato- 

 zoon is seen within the oocyte on the right side of the figure. Meanwhile 

 the oocyte is becoming a mature ovum by undergoing divisions and pro- 

 ducing the second polar body; and the anaphase of this division is shown in 

 Fig. 24, A. Sobotta stated that no centrosomes occur in connection with 

 the spindles of the maturation divisions, and Long and Mark have like- 

 wise failed to find any "typical centrosomes." 



In Fig. 24, C, the second polar body has become a separate cell. The 

 chromosomes of the ovum, which is now mature, have formed a compact 

 mass. They next become resolved into a chromatic reticulum, and a 

 resting nucleus is produced, provided with a nuclear wall and distinct nu- 

 cleoli (Fig. 24, D and E). This nucleus, which becomes large and moves 

 toward the center of the cell, is known as the female pronucleus. Mean- 

 while the head of the spermatozoon has enlarged and formed the male 

 pronucleus, as shown in Fig. 24, C, D and E. 



The two pronuclei, which are very similar, develop rapidly, "probably 

 within a few minutes after the entrance of the spermatozoon." Simul- 

 taneously they prepare for division, and the chromatic reticulum of each 



