142 A. RICHARDS. 



germ cells from the differentiation of the ovarian anlage to the 

 formation of the membrane of the ovary the term " pre-oogonia " 

 has been (aptly, I think) suggested by Professor Dahlgren (see 

 Figs. 9-14). It is only at the end of the pre-oogonial period that 

 the cells which are to form ova are certainly distinguishable 

 from all others. The convenience of the use of this term will be 

 apparent. The formation of the follicular membrane seems to 

 mark the time of cessation of active proliferation on the part of 

 the oogonia. I have seen no sign whatever of cell division in 

 stages following this. There are no longer many parenchymal 

 elements to be seen in the ovary and each oogonium becomes 

 sharply marked off from its neighbors. The subsequent growth 

 of the ovary is merely that attendant upon the growth period 

 of the oogonia. The cell divisions are, of course, limited to the 

 maturation and cleavage divisions. 



The Growth Period and Behavior of the Nudeolus. The growth 

 period in the cestode as in most animals is relatively long; many 

 more proglottids have the ova in this condition than in any 

 other previous to the cleavage stages. The actual duration of 

 the growth period is probably three or four months. 1 It is 

 well known that sheep older than yearlings are rarely infested 

 with tape-worms. The parasites occur only in the young, but 

 nevertheless they attain a length of 4-5 meters in the case of 

 M. expansa and of 2 meters in the case of M. planissima. De- 

 veloping embryos first occur at a distance of 100-110 cm. from 

 the head; the primary anlage appears 7-8 cm. from the head; 

 and the growth period of the oogonia begins about 20 cm. from 

 the head. This short duration of the stage in which multiplica- 

 tion of the oogonia is taking place must be kept in mind when 

 investigating the method of cell division here. To Child's de- 

 scription of the growth period little can be added. Briefly, there 

 is accomplished during the period the great growth of the eggs, 

 synapsis, the formation of the yolk globules, and certain con- 

 temporary changes in the nucleolus to be described immediately. 

 During this period of no divisions the oogonia after synapsis 

 are characterized by a minimum amount of chromatin, faintly 

 staining, distributed peripherally, although strands may reach 

 the nucleolus. 



1 These time limits are tentative for I have been able to approximate them, only 



