12 



THE MAMMALIAN EGG 



oocyte grows and so a wide band of follicle cells is formed. Growth 

 of the oocyte proceeds until it has increased its original volume, 

 both of yolk and cytoplasm, many times. Follicular enlargement 

 continues long after the oocyte has reached its maximum size; this 

 growth is attributable partly to further multiplication of follicle 

 cells, but chiefly to the formation of a fluid-filled space or antrum 

 within the follicle. Throughout all these changes, the oocyte nucleus 

 remains in the dictyate stage of the first meiotic division. Then, at a 

 set time before ovulation is due, the meiotic division is suddenly 

 resumed, the first polar body is emitted and the egg becomes a 

 secondary oocyte. As a general rule, ovulation occurs spontane- 

 ously, but in some animals (Table i) it is induced by the act of 

 coitus. In most species, the egg is ovulated as a secondary oocyte 

 and does not mature further until it is penetrated by a spermatozoon. 



In the dog, fox and possibly the horse, 

 however, the egg enters the Fallopian 

 tube while it is still a primary oocyte 

 (Van der Stricht, 1923 ; Pearson and 

 Enders, 1943 ; Hamilton and Day, 

 1945); in the dog, sperm penetration 

 can occur at this stage, but generally 

 takes place during the first meiotic 

 division (Fig. 8) or at the beginning of 

 the second. (Ovulated oocytes are 

 known also in rats and mice; they do 

 not appear to be fertilizable though 

 spermatozoa may pass through the 

 zona pellucida: Austin and Braden, 

 1954c.) After sperm entry, the second 

 meiotic division proceeds, the second 

 polar body is emitted and the egg is 

 now known as an ootid, a term that applies throughout fertilization. 

 When the chromosome groups deriving from the male and female 

 pronuclei have come together, fertilization is regarded as complete 

 and the cell is called a zygote. With successive mitoses, the egg 

 divides, first into two cells, then into four, eight, sixteen cells, and 

 so on, until the egg, or embryo as it is now more often called, 

 comes to consist of a spherical mass distinguished as a morula. 

 Finally, a space appears within the morula and grows in volume; 

 this state characterizes the blastocyst, and it is as such that the 



Fig. 8 



Drawing from an illustration by 

 Van der Stricht (1923) of a dog egg 

 with a sperm head lying near the 

 metaphase first-maturation spindle. 



