828 



DEVELOPMENT 



[CH. LIX. 



B 



FIG. 620. Diagram showing the formation of the 

 polar bodies (maturation of the ovum). A, B, 

 and C show stages in the formation of the first 

 polar body by heterotype mitosis. A is the 

 ooycte of the lirst order at the commencement 

 of mitosis, when only half the usual number 

 of chromosomes appear. C is the oocyte of 

 the second order ; it has no distinct nucleus, 

 because no resting-stage occurs ; after the 

 separation of the first polar body, the chromo- 

 somes which remain in the oocyte of the second 

 order at once rearrange themselves on a new 

 spindle. D is the. mature ovum, with the 

 female pronucleus and the two polar bodies. 

 1, First polar bud; 2, first polar body; 8, 

 second polar body ; 4, chromosomes on spindle 

 of oocyte of first order ; 5, zona striata ; 6, 

 vitelline membrane ; 7, daughter chromosomes 

 in first polar bud ; 8, female pronucleus. 



cicatricula, about J of an inch in 

 diameter. In the cicatricula lies 

 the nucleus or germinal vesicle, 

 and it is this small mass of proto- 

 plasmic substance which divides 

 and grows to produce the chick; 

 the yolk and the surrounding white 

 being used as food. 



Ova like the hen's, in which 

 only a small part, the cicatricula, 

 divides and grows, are called mero- 

 blastic. Small ova, with little food 

 yolk, such as the human ovum, 

 divide completely during develop- 

 ment, and are called holoblastic, 

 but numerous gradations occur 

 between the two extreme types. 



The structure of the mammalian 

 ovum and its mode of formation 

 have already been considered 

 (p. 824), but before such an ovum 

 can develop it must first become 

 mature, and then it must be im- 

 pregnated by the entrance of a 

 spermatozoon. 



Maturation of the Ovum. 



It will be remembered that the 

 germ cells which form the ova 

 are at first embedded in the ger- 

 minal epithelium, from which they 

 pass into the stroma of the ovary, 

 and then by division and growth 

 they form oogonia ; from the 

 oogonia, oocytes of the first order 

 are developed, and the oocytes of 

 the first order become enclosed in 

 Graafian follicles. It is the process 

 by which the oocytes of the first 

 order become con verted into mature 

 ova, which is known as maturation, 

 and it consists essentially of a 

 double mitotic division of the 

 oocyte, each division producing two 



