G. A. DREW ON YOLDIA LIMATULA, SAY. 23 



ganglia eg, arise, are separated externally by test-cells, but internally 

 they are, and continue to be, connected by cells that do not take part in 

 the formation of other organs. 



The other internal cells have increased in number, and their 

 origin can no longer be determined. At a little later stage, Fig. 42, 

 the dorsal internal cells are found to be arranged in a layer, sg, that 

 represents the beginning of the formation of the shell-gland. Other cells 

 soon arrange themselves over the remainder of the surface inside the 

 test and, together with the shell-gland, form a new ectoderm. This 

 ectoderm probably is formed from cells that were originally surface cells. 

 If this view is true, the original surface cells give rise to at least three 

 groups : those that remain at the surface, acquire cilia, and become 

 differentiated as test-cells ; those that form the cerebral ganglia and 

 the apical plate ; and those that form the new ectoderm inside the test. 



In the stage of which Fig, 42 represents a section, a space seems 

 always to be present in about the same position. This space probably 

 represents the beginning of the lumen of the mid-gut. 



The embryo continues to elongate, Fig. 32, becomes cylindrical, and 

 the test-cells flatten and become less distinct from one another. 

 Between about the 32nd hour and about the 70th hour, there is no 

 appreciable change in the external appearance of the embryo and, as 

 the test-cells are quite opaque, few internal changes can be followed in 

 living specimens. 



The test, which is now fully formed, is composed of large vacuolated 

 cells, Fig. 44, t, the nuclei of which are almost, if not quite, in contact 

 with their inner walls. Just beneath each band of cilia the protoplasm 

 stains very deeply. Just inside the test, between it and the new 

 ectoderm, there are, frequently, a few 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 mid-gut, 

 Fig. 43, becomes surrounded by a definite wall of rather large cells, and 

 an invagination, std, extends into the mass of cells from the ventral 

 side of the blastopore. This invagination is the beginning of what I 

 have called the ventral tube,* but, as it has since been found that it 

 is formed as an ectodermal invagination that does not at first 

 communicate with the mid-gut, a special name is unnecessary, and it 

 will hereafter be referred to as the stomodasum. 



* Johns Hopkins University Circulars, No. 132, 1897. 



