509 



face. Between the ectoderm and test there is, frequently, a thin film 

 of protoplasm containing a few scattered nuclei. 



The dorsal ectoderm cells enlarge and form the shell -gland and, 

 soon after, an ectodermal invagination, in the region of the blastopore, 

 forms the stomodoeum. The stomodamm comes to lie on the ventral 

 side of the embryo just inside the test. It is to this tube that I pre- 

 viously 1 ) gave the name of ventral tube, but, as I have since found 

 that, in Yoldia, it is ectodermal in origin, there is no reason to give 

 it a special name. 



Fig. 13. 



Fig. 14. 



Fig. 13. Transverse section of a 45 hour embryo of Yoldia limatula, taken just in 

 front of the second band of cilia, tng mid-gut. sg shell-gland, std stomodseum. t test. 



Fig. 14. Transverse section of an embryo of Nucula delphinodonta, in which the 

 mantle is just beginning to form. The section is taken about mid -way of the embryo. 

 tn mantle, mg mid-gut. std stomoda?um. t test. 



The stomodaeum, Figs. 11, 12, 13 and 15 std, pushes in, and joins 

 the mid-gut, mg, which is formed in the anterior portion of the em- 

 bryo. In the meantime in Y. limatula and N. proxima, the shell-gland 

 has attained its greatest invagination, that is it has become slightly 

 concave, Figs. 11 and 12, sg. In N. delphinodonta it flattens but does 

 not become concave. In all, the shell -glands soon arch upward pre- 

 paratory to protruding laterally to form the mantle, Fig. 14 m. 



The cells from which the cerebral ganglia of Yoldia are formed, 

 arch inward and form two pouches that lie side by side. These pouches 



1) Johns Hopkins Univ. Circulars, No. 132, 1897. 



An.it. An/., XV. Aufsätze. 



34 



