20 



HISTOLOGY 



in which the somatic cells are so strongly and so early differentiated 

 from the reproductive cells that they cannot retain their power of 

 reproduction and perform somatic duties at the same time. 



In the first examples the groups of somatic cells are usually also 

 slightly differentiated from each other, while in the latter they are highly 

 differentiated. 



This differentiation does not all come about at once. The dividing 

 oosperm may transmit to its descendants all of its qualities and original 

 powers and structures for a considerable number of cell generations 

 before any one of these descendants begins to differentiate. Or the dif- 



FiG. 17. A, second cleavage division of the oosperm of Ascaris, showing the first differentia- 

 tion by loss of chromatin in the somatic cell. B, resulting four cells, showing the lost chro- 

 matin, ch., and the smaller resulting nuclei in the daughter somatic cells. (From WILSON 

 after BOVERI.) 



ferentiation may begin by changes in one or the other of the two cells 

 produced by the first cleavage division. In fact the frequent greater 

 size of one of these first two daughter cells of the oosperm shows that 

 there were differentiating forces in the oosperm itself Before it began to 

 divide, and we are thus brought to see that the beginnings of differen- 

 tiation are sometimes preformed in the oosperm. This can be seen in 

 a number of ways in the developing eggs of several kinds of animals and 

 may be discussed under a few principal headings. 



The simplest and most fundamental form is that seen in the organisms 

 whose oosperms show a distinct polarity in their organization, as in the 

 frog and many other animals. A feature of this polarity is the collec- 

 tion of the yolk or food supply of the ovum at its lower end and of the 



