3l6 The Oogenesis of the Tortoise 



contraction- by which the microsomes are made to approach one another. 

 The circle of microsomes may thus become closely applied to the central 

 granule, giving rise in that way to the conspicuous centrosome which 

 appears when the oogonium divides. 



Division of Oogonia. 



The marked difference in size of the oogonia must mean that they 

 grow considerably before dividing to form oocytes. This growth con- 

 sists in a marked increase in size of the nucleus and the greater amount 

 of chromatin or at least chromophilous granules, as well as in a marked 

 increase in the amount of cytoplasm. During this growth the radial 

 zone of the sphere persists (Plate I, Figs. 1, 2, 3). 



The division of the oogonia is mitotic (Plate I, Fig. 3). The chromatin 

 becomes massed into a spireme, a spindle is formed with a centrosome at 

 each pole of the spindle. I have no observations on the division of the 

 centrosome to record. But I presume that the two centrosomes result 

 from division of the original centrosome. This fact may be noted, how- 

 ever, that the centrosomes are now very much more conspicuous than 

 the original centrosome, due, as it seems to me, to the tension of the 

 astral system, the massing of the archoplasm around the centrosome and 

 the concentration of the circle of microsomes so as to make them seem 

 merged into the central granule. 



When an oogonium enters the division period, it passes through a 

 series of divisions with a very brief period, if any, between each division 

 (Plate I, Figs. 2, 3). Thus it first divides into two (Plate I, Fig. 6). 

 The two cells thus formed (Plate I, Figs. 8, 9) divide again immediately 

 after the reconstruction of the nuclei, giving rise to four similar cells 

 (Plate I, Fig. 9). After a brief interval, these again divide, giving 

 rise to eight daughter-cells, from the original oogonium (Plate I, Fig. 

 10). As might be expected, the eight cells thus formed are very small 

 as compared with the original oogonium (Plate I, Fig. 3). As can be 

 seen in Plate I, Fig. 3, not only are the spindles associated into groups, 

 but the progeny of each oogonium lies crowded together in nests of two, 

 four or eight cells, and are surrounded, as was the mother-cell, with the 

 inner layer of the stroma cells. 



Formation of Follicle. 



One of the eight cells, resulting from the repeated division of the 

 oogonium, becomes the oocyte or egg; the rest become the follicle. The 

 follicle cells are, therefore, the sister-cells of the Qgg. The oocyte is 



