EARLY STAGES OE DEVELOPMENT OF THE BABBIT. 133 



Through the remaining 120° the inner mass is still as much part 

 of the wall of the vesicle as the outer layer of cells. 



In fig. 23 the cavity shows a further increase in size. The 

 outer layer of cells (0. L.) now show signs of having become 

 stretched, due, I believe, to the rapidly increasing hydrostatic 

 pressure within the blastodermic vesicle. The walls of the 

 vesicle are now, while living, firmly applied to the zona radiata 

 and albumen layer. The space shown between the vesicle and 

 the zone in the figure is due to reagents. The same remark 

 applies to all the figures from the 22nd onwards. 1 have not 

 been able to distinguish sharp lines of division between the 

 outer layer cells in section after the commencement of the 

 blastodermic vesicle cavity. In surface view there are certain 

 lines of divisions, which, as van Beneden has shown, are very 

 clearly brought out by silver nitrate. 



In this fig. 23, which I believe to be a median section, there 

 is a fairly clearly defined line marking the inner mass from the 

 outer layer, which is less perceptible in fig. 22, and which 

 becomes very distinct in later stages, such as in fig. 28. 



It is now possible to distinguish clearly an inner mass as 

 separate from the outer layer. This separation seems to be 

 caused simply by the tension being more acute in the outermost 

 of the segments of the mass /. M. in fig. 22, causing these seg- 

 ments to be more stretched than the remainder, because they 

 are more directly united with the already separated and stretched 

 cells O. L. 



Lines of division between the segments of the inner mass 

 can usually be found, but varying greatly in definition. I 

 cannot say to what extent these several segments may be really 

 divided. The inner mass is certainly connected in some way 

 with the outer layer, but whether by direct protoplasmic 

 union 1 am quite unable to say. 



Fig. 24 presents no new characters excepting a tendency to 

 flattening of the inner mass. In figs. 25, 26, 27, the inner 

 mass is flattened out still more, so that it presents a lenticular 

 form in section instead of a circular outline as it did in figs. 

 22 arid 23. How does this change in form come about ? 



