4 WILLIAM A. HILTON. 



the cavity formed by ua-trulation. Fig. 4 is through a specimen 



-uch a lau-r stage where the total cavity, although slight, is 



ir and definite. The lining of the first part of this is from 



invaginated cells and some of the original ones which were with 



small yolk granule- on the animal pole may be distinguished as 



part of the lining of the cavity, the total extent of which is shown 



in a few sections bevoml this level. 



FIG. 5. Longitudinal section through blastopore and archenteron showing 

 dorsal and ventral lip. Small cells with small granules dotted as in the other 

 drawing. 



In later stages the cavity or archenteron may not be very 

 extensive. Cells just under the dorsal ectoblastic ones maybe 

 seen to be composed of small yolk granules and beyond the point 

 where in vagi nation is apparent these seem to be well organized 

 and represent a part of the middle germ layer and were probably 



-.. ; ; 7V.-. .;._..-.;. 



rvy - ' ; "'". 

 ' ' ' ' 



FIG. d. Longitudinal -i-rtinii through the lila-top.ire of a stage a little later 

 than that oi Fig. 5, showing cells which have grown in from the dorsal lip. X-'5- 



formed from some of the earlier sin, ill cells which wen- traced 

 in their migration-, about the yolk mass. 



In other specimens the cavity of the archentcnm is not very 

 wide, mosl of the d'M-.il cells lining it contain small yolk granules, 



