DEVELOPMENT PRIOR TO LAYING 59 
germ-wall by the area with large granules Y. These zones con- 
stitute the opaque area. The circles in the pellucid area represent 
megaspheres, that is yolk-masses cut off from the floor of the 
subgerminal cavity and lying in the latter (cf. Fig. 28 M). The 
invaginated entoderm is represented by the crossed area E; 
the lip of the blastopore, where ectoderm and entoderm are 
continuous, by the region R. 
Fig. 31. — A diagrammatic reconstruction of the blastoderm repre- 
sented in Fig. 30. (After Patterson.) 
R., Mass of cells left after closure of blastopore. S.G., Anterior 
portion of subgerminal cavity not yet crossed by the entoderm. Other 
abbreviations as in Fig. 29. 
The last three or four hours prior to laying witness the closure 
of the blastopore. A comparison of Figs. 27 and 29 will show 
that the blastopore has become considerably narrower in the 
later stage. It will be observed that the posterior ends of the 
germ-wall are approaching. Finally they come into contact, and 
the blastopore is closed. During this process the lip of the 
blastopore is not cut off externally, but on the contrary comes 
