STUDIES ON GERM CELLS 393 



p.g.c.) divides by mitosis resulting in two oogonia (fig. 33, oogi) 

 which he at the posterior end of the egg. Each of these divides 

 again about the time when the blastoderm cells are cut off by 

 cell walls. Four oogonia of the second order (fig. 34, oog^) are 

 formed in this way. A third division results in the production 

 of eight oogonia of the third order (fig. 35, 00^3). The germ-band 

 then forms and segments, and the eight oogonia are passively 

 carried around by the growth of the tail fold as shown in figure 

 36, ooQs). The embryo then grows broader and shorter until 

 it entirely surrounds the yolk and the end of the tail fold coincides 

 with the posterior end of the egg. During these developmental 

 stages the oogonia remain undivided, but become separated into 

 two groups of four each, which lie in two rows, one on either side of 

 the embryo in the region of the eleventh segment (fig. 37, 00^3)- 

 Soon each row of four oogonia becomes enclosed by mesoderm 

 cells, forming an ovary. The germ-glands then become almost 

 spherical and soon the oogonia undergo a division by mitosis, 

 thus forming eight oogonia of the fourth order in each germ- 

 gland (fig. 38, ooQi). These divide again by mitosis (fig. 38, a) 

 producing sixteen oogonia of the fifth order (fig. 39, 00^5) in each 

 germ-gland. The final division of the oogonia takes place shortly 

 before the larva hatches. 



Typically, there are then thirty-two oogonia of the sixth order 

 in each germ-gland, but in some cases certain of the oogonia of 

 the fifth order are prevented from dividing. All of the oogonia 

 do not produce embryos, since, as a rule, only from five to seven- 

 teen larvae are produced by a single mother-larva. ' The oogonia 

 of the sixth order grow into oocytes (fig. 40, 00c.) ; each of these, 

 together with a syncytium containing about twenty-four nurse- 

 cells of mesodermal origin (fig. 27, n.c), becomes surrounded by 

 follicular epithelium (fig. 27, f.ep.) also of mesoderm cells. Dur- 

 ing this process the nucleolus of the germ-cells disappears and 

 the chromatin forms long slender threads (fig. 27, ooc.n). This 

 completes the history of the germ-cells from one generation to 

 the next. The accompanying diagram (fig. 5) shows graphically 

 the germ-cell cycle in this animal. 



