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HUBERT DANA jGOODALE 



h sides of the blastopore. I have described the blastopore 

 as shifting, but this was merely' a convenient view point. In 

 reality it is due to the shifting of cells relative to the blastopore, 

 i.e., the opening left by infolding cell masses. 



There is one apparent correlation between this shifting of the 

 blastopore and sections. Fig. 36, a cross-section of an egg some- 

 where about this same period of development, shows at the level 

 of the archenteron floor at the corners a mass of loose cells. Sim- 

 ilar sections through other eggs at this period often show loose 

 cells on one side only of the egg, (fig. 37, right), the cells on the 

 other side, having already become united. The row of cells 

 lining the archenteron at the corners are usually united into a 



74 



75 



76 



77 



Figs. 74-77 



layer (fig. 36, left side). The separated cells lining the archen- 

 teron at the right (fig. 36) are not found as frequently. Now, 

 the forerunners of the ventral mesoderm and the posterior part 

 of the gastral mesoderm dorsal to the blastopore, lay at the equa- 

 tor of the egg before gastrulation began. It is not unreasonable 

 to suppose that the mesoderm cells which are to lie in the vacant 

 places shown in fig. 36, also lay about the equator of the egg. 

 The only other assumption necessary is to suppose that these 

 cells are not invaginated at exactly the same time the rest of the 

 cells are, but that they move somewhat independently of the 

 general process. It is easy to derive a large part of the mesoderm 

 cells from the floor cells of the blastocoele, since they can be seen 

 actively migrating into i)ositions where they must eventually 

 become mesoderm. The yolk-cells of the equatorial region are 

 naturally somewhat less free to pursue an entirely independent 

 course to their final position. 



