462 



HISTOLOGY 



primary oocyte has but 6 chromatin masses, which are larger, however, 

 than usual, and in some animals can be seen to consist of four portions 



each. In this latter 

 case they are called 

 the tetrads. 



A mitotic division 

 now takes place in 

 which the tetrads are 

 split in two, and each 

 daughter cell re- 



FIG. 428. Astenas rorbesii. Region of young ovary from . , . . , 



which the reproductive cells originate. X 1500. (Drawn CClVeS dyads. 1 he 



by H. E. JORDAN.) two resulting cells are 



known as oocytes of the second order, and they at once proceed to perform 

 another mitotic division without, meanwhile, re-forming the nucleus. 

 This second division results in the dyads being pulled apart and each 

 of the four resulting cells getting 6 monads, or 6 chromosomes, as we must 

 term them. This is one half the somatic number. One of the 4 cells 

 is now a matured ovum. 



These equal divisions of the nuclear elements were not followed by 

 equal divisions of the cell body and its load of yolk. When the primary 

 oocyte divided, one secondary oocyte took practically all the cytoplasm, 

 leaving its sister cell to appear as a tiny mass of nuclear substance which 

 is discharged from the ovum. This smaller secondary oocyte is called 

 the first polar body. In the second reduction division the same thing is 

 repeated, and one of the resulting ova is discharged (divided) from its 

 sister cell as the tiny second polar body, while the first polar body often 

 makes an attempt at division which results in there being three polar 

 bodies attached to an ovum instead of two. 



This process, especially its latter part, can be very easily traced in 

 the growing and maturing ova of a starfish, Asterias Forbesii. At an 

 early age the reproductive cells 

 of this animal are situated in 

 the epithelium lining a compound 

 tubular organ, which is the ovary. 

 They are exceedingly small and 

 can only be detected among the 

 somatic cells when they begin to 

 grow in size for the maturation 

 process. Figure 428 shows them 

 just before this occurs. It occurs 

 principally during a few spring 

 and summer months in Asterias Forbesii, but goes on to some ex- 

 tent during the whole year. At this time the ova begin to enlarge and 



FIG. 429. Asterias Forbesii. Group of young 

 oogonia (ogn.). Three of the oogonia are un- 

 dergoing synizesis (s yn.~). X 1500. (Drawn 

 by H. E. JORDAN.) 



