42.. 



The embryo continues to elongate, figure 32, becomes 

 cylindrical, and the test cells flatten and becone less distinct 

 from one another. Between about the 32nd hour and about the 

 70th hour, tliere is no appreciable change in the external appear- 

 ance of the onbryo and, as the test cells are quite opaqiie, few 

 internal changes can be followed in living specimens. 



The test, iiibich is now fully formed, is coiirposod of 

 large vacuolated cells, figure 44, t, the nuclei of which are al- 

 most, if not quite, in contact with the inner walls. Just be- 

 neath each band of cilia the protoplasm stains very deeply. Just 

 inside the test, between it and tiie new ectodei^, are, frequently, 

 scattered nuclei, lying in a very thin film of protoplasm. Their 

 significance is not known. 



The shell gland becomes more definite, the lumen of the 

 midgu':, figure 43, mg, becomes surrounded by a definite wall of 

 rather large cells, and-imrraginat ion, std, extends into the mass 

 of cells from the ventral side of the blastopore. Thtis invagina- 

 tion is the beginning of what I have called ^-) the ventral tube, 

 but, as it has since been found that it is foimed as an ectodermal 

 invagination that does not at first communicate with the midgut 

 there seems to be no reason to give it a special name, and it 

 will hereafter be referred to as the stomodaeum,^ 



