THE EMBRYOLOGY OF A SCOKPION. 109 



staining nucleolus. There is at first no sign of its presence 

 on the outside of the ovarian tube. As it increases in size, 

 however, it pushes its way, at the head of a column of cells, 

 towards the outside. The outer layer of the ovarian tube 

 becomes very thin, but remains as a membrane containing 

 few, if any, nuclei (fig. 2, fol'.). By the time the ovum is 

 about '04! mm. in length (fig. 2) it has passed completely 

 through the outer layer and is visible as a small protuberance 

 on the surface of the ovary. It remains connected to the inner 

 layer of the tube by a column of cells which is somewhat 

 expanded over the base of the ovum. The nucleus has not 

 increased in size in proportion to the growth of the cell. 



The nuclei of the cells of the column which connects the 

 ovum to the inner layer of the ovarian tube next arrange 

 themselves so as to leave a clear space down the centre of the 

 column (PI. XIII, fig. 3, mi.). They also grow round the ovum 

 so as to form a follicle (fig. S,fol.) one cell thick. The 

 cells of this follicle rapidly become flattened and their nuclei 

 become smaller. The cells which remain clustered at the base 

 of the ovum (fig. 3, ger'.) on the other hand increase in size, 

 and shortly after the stage represented in fig. 3, which is a 

 drawing of an ovum of about •! mm. in diameter, they begin 

 to secrete the yolk of which the greater part of the ripe egg is 

 composed. The outer layer of the ovarian tube can still 

 be traced as a thin and apparently structureless membrane 

 (fig. 3, foV.) surrounding the egg outside the cellular follicle. 

 The nucleus has increased in size and is now a distinct oval 

 body with a round, granular nucleolus. 



In fig. 4 is shown a longitudinal section of an egg of about 

 •4 mm. in length and "28 mm. in breadth. A considerable 

 quantity of yolk is now present in the form of spheres 

 ranging in size from mere granules up to as much as "025 mm. 

 These spheres are clear, homogeneous, sharply defined bodies 

 showing no internal structure except that there is, in the 

 larger ones, a collection of granules at one point near 

 the outside. Round the nucleus the yolk-spheres are small, 

 and round the margin of the egg the protoplasm is coarsely 



