DEVELOPMENT OF PARACOPIDOSOMOPSIS 27 



5. HISTORY OF THE POLYGERM 

 a. Multiplication of the primary masses 



In the completed polygerm each primary mass consists of 

 several embryonic cells surrounded by a relatively thick inner 

 membrane, and the various primary masses are more or less 

 separated from one another by ingrowths from the polar membrane 

 (fig. 58) . Soon after the polygerm is formed, the primary masses 

 begin to multiply by fission. The division is initiated by a 

 constriction of the inner membrane, followed by a corresponding 

 constriction or ingrowth of the polar membrane. In figure 

 59 some of the primary masses are beginning to divide, and in 

 figure 60 the one on the left is in the act of dividing. 



In the description of these stages we shall refer to the prod- 

 ucts of division of the primary masses as secondary masses, 

 and when these in turn divide their products will be referred 

 to as tertiary masses. In later stages the tertiary masses also 

 divide a number of times, but such products will be called com- 

 ponents, whenever they can be distinguished from the ordinary 

 tertiary masses. During the late history of the polygerm it is 

 not always easy to determine whether a given mass is secondary 

 or tertiary. Their general structure is the same and both kinds 

 are frequently present in the same polygerm, owing to the fact 

 that the divisions do not occur simultaneously. However, one 

 can usually distinguish the two kinds of masses by their dif- 

 ference in size. 



Figure 61 is a detailed drawing of a section passing through tw^o 

 secondary masses that have recently been formed. The mass 

 on the left has received a single embryonic cell, while the one on 

 the right has received three such cells. The general rule is for 

 a secondary mass to have several embryonic cells, but occasion- 

 ally only a single cell is included. 



Figure 62 passes through the middle of a polygerm in which 

 several of the primary masses are dividing. Some of them have 

 already completed the division (fig. 62, S.M.). At the points 

 marked X, Y, Z, are three primary masses in different stages of 

 division. In all these cases the division has been accomplished 



