42O 



HISTOLOGY 



reproductive elements are called the nurse cells and they are usually 

 former reproductive cells that at one stage or another of their develop- 

 ment have given up their career as reproductive cells and devoted their 

 energies to the nourishment of their more fortunate neighbors. The 

 other gonad structures are the same tissues that are found in most 

 organs, as connective, blood, muscle, and nerve tissues. They play the 

 same part here as in the other organs. 



Origin of the reproductive cells. As stated before, the reproduc- 

 tive cells are the original and unaltered descendants of the dividing 

 oosperm, the only ones of these descendants that have not been differ- 



FiG. 379. A, second cleavage division of the oosperm of Ascaris, showing the first differen- 

 tiation by loss of chromatin (ch.) in the somatic cell. B, resulting four cells, showing the 

 lost chromatin, ch., and the smaller resulting nuclei in the daughter somatic cells. (From 

 WILSON after BOVERI.) 



entiated. This does not mean that changes of size and of arrangement 

 of the cell-organs have not taken place, but it does mean that the cell 

 has retained all its original powers and the necessary structures to exert 

 these powers, which is not true of the somatic cells. 



This loss of power by the somatic cell can even be demonstrated, in 

 one case at least, to be a loss of some part of its chromatin and to occur 

 in some species in the first cell division or shortly after. Such a solitary 

 case of actual proof exists in the nematode worm, Ascaris, in which Boveri 

 has clearly shown that, at the first division of the oosperm, one of the 

 resulting cells is slightly differentiated from its sister cell by a potential 

 loss of chromatin that occurs in the next division, while the other cell 

 retains every feature of its parent, the oosperm (Fig. 379). 



