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HISTOLOGY 



FIG. 443. Prophase of the first matu- 

 ration division in a mouse ovum. 

 (After SOBOTTA.) 



are still to be found the unchanged oogonia, now become nurse cells. 

 The pre-oocytes occur either singly or in groups of two and three or more. 



When another ovum is to be matured, 

 one of the single cells or one of the 

 groups moves down and becomes a 

 primitive follicle. If more than one pre- 

 oocyte is in this primitive follicle, one 

 of them develops at the expense of the 

 others, and it is rare to find a follicle 

 containing more than one ovum at ma- 

 turity. In some other mammals very 

 different histological conditions obtain 

 during this history, especially in such 

 as have no great mass of connective 

 tissue in the ovary. 



The development of the follicle has 

 already been spoken of. The nurse 

 cells form a single layer around the ovum and then stratify to form 

 a double layer, which continues to multiply its cells by mitosis until 

 there is a thick multicellular layer. 



Finally, the layer which is directly around the ovum splits concen- 

 trically from the layer which lines the connective-tissue capsule of the 

 follicle, and the two become separate except at one point, called the hilum. 

 When the ovum is ripe, the capsule and the outer wall of the ovary 

 break, and the egg is discharged into the body cavity, whence it passes 

 into the oviduct and down to the 

 uterus. 



From the time that it leaves 

 the layer of pre-oocytes, the re- 

 productive cell undergoes a rapid 

 growth, and at or about the time 

 it is discharged from the mature 

 follicle, it undergoes the two 

 maturation divisions. The cat 

 does not easily exhibit this part 

 of the process, and so we shall 

 turn to rodent material to exam- 

 ine these in the mammal. 



The first maturation spindle 

 in the maturing ovum of the 

 mouse is composed of distinct 

 fibers without astral rays. Despite Sobotta's claim that the spindle 

 fibers do not converge but tend to lie parallel, as straight lines from pole 



FIG. 444. Anaphase of first maturation division 

 in the muskrat, Fiber zibethecus. X 750. 



